The movie begins by showing the Warner Brothers and Village Roadshow logos embedded into a cobblestone pavement. We transition to Dr. John Watson (Jude Law) and Inspector Lestrade (Eddie Marsan) riding in a carriage in the middle of the night. Sherlock Holmes (Robert Downey Jr.) follows on foot. He's in a tremendous hurry - he darts between columns, up and down stairs and around buildings effortlessly - and finally enters a non-descript building.

Once inside, Holmes starts running down a spiral staircase, but pauses when he notices a henchman waving a lantern and keeping watch. He analyzes the situation, then takes out the henchman. We see the attack get carried out twice - first as Holmes plans it, and again as he executes it: a hammer blow to the side of the head, then grabbing the henchman by his neck to silence his scream, then dragging his left leg. Then in real-time, we see Holmes do all of the following at rapid speed before stealing the sentry's bowler hat. Holmes continues running down the stairs, until he reaches the basement, where a black magic ritual is taking place. Holmes hides behind a column to assess the situation again.

In the center of the room, he observes a young woman wearing a white dress, tied to a table, while a hooded figure stands over her chanting some sort of Latin. Scattered across the room are several other hooded figures and henchmen. Holmes begins calculating how to take them out, but is interrupted when a guard comes up from behind him. He fights with the guard, then Watson shows up and chokes the man into unconsciousness. They greet each other with amusing banter (their friendship is very much written as a bromance, and come up at several points in the film), where Watson chides Holmes for forgetting to bring his pistol AND forgetting to turn off the stove.

Watson and Holmes race downstairs and attack the henchmen, overpowering most of them while the hooded figure continues the ritual. Back at the table, the (possibly possessed) girl reaches up for a dagger and makes to stab herself. Holmes finishes defeating the henchmen, hurries over and stops her just in time. The hooded figure stops to greet Holmes by name, and is unmasked as Lord Henry Blackwood (Mark Strong). As he taunts Holmes, Watson comes running over, but is stopped by Holmes. Turns out, Lord Blackwood is holding some kind of thin stiletto glass knife that would have pierced Watson through the nose if he had gotten any closer. Holmes directs Watson to put his energies into tending the girl, and Lestrade and his men burst in just in the nick of time.

As Blackwood and the henchmen are arrested, Lestrade chides Holmes for not waiting for Lestrade's orders. Holmes says that the girl's parents hired him, so he doesn't report to Lestrade. Before Lestrade can retort, a newspaper photographer who wants to take their picture interrupts him. Holmes throws up an arm and prevents the camera from capturing his face. Thus, all the credit is given to Lestrade instead.

The credits flash by and consist of newspaper headlines detailing Holmes and Watson's exploits.

Three months later, at 221B Baker Street, Watson is treating an elderly patient. As he dresses, the patient asks about Watson's plans to move his medical practice to a new headquarters. Watson confirms that he is moving, and that he hopes to have a woman's touch around the place soon. The patient congratulates him on his (potential - as Watson hasn't proposed yet) nuptials, before nervously asking if Holmes is moving too. Watson says no, but is promptly interrupted by several gunshots that send both men ducking for cover. The patient leaps up and says that the blasts must be gunfire, but Watson soothes him and lies that Holmes is probably hanging a picture with nails and hammer. Watson ducks out to check on the commotion, and is met by Mrs. Hudson (Geraldine James). She tells Watson that Holmes is in a mood, and she hopes that he can calm Sherlock down. At this point, the elderly patient comes out and is about to talk, when there's the sound of gunfire again. Watson tells Mrs. Hudson to get the patient a cup of tea, and he'll go see to Holmes himself. He also asks Mrs. Hudson to bring some food to cheer up Holmes.

When Watson walks into Holmes's study, the entire room is dark. Watson comments into the darkness that he knows Holmes is bored, as it's been awhile since the Blackwood case. Holmes remarks that he's trying to figure out a way to silence the sound of gunfire, and Watson sarcastically retorts that it's clearly not working. As Watson talks, he goes around opening up the blinds in the room, which causes Holmes to groan painfully.

Watson starts rifling through Holmes's mail and offers him prospective cases to consider. Holmes flippantly solves all of them without a second thought. Watson also points out that Holmes is in the papers again, as Lord Blackwood is about to be hanged, and Watson will be the attending physician, as he considers it a good away to conclude his final adventure with Holmes.

Mrs. Hudson comes in with tea and snacks, points out that the family bulldog Gladstone is lying immobile on the floor, saying that Holmes has killed the dog, again. Watson hurries over to check on the animal, and asks Sherlock what he's done to the dog now. Holmes nonchalantly says he was just testing out a new anesthetic.

Nothing seems to be cheering Holmes up, so Watson throws down the mail and tells Holmes that he needs to get out of the house. He tells Holmes that he's going to join Watson and his future fiance for dinner at a London hotel. Holmes begrudgingly agrees.

At an upscale restaurant, Holmes checks his watch several times before Watson and Mary Morstan (Kelly Reilly) show up. They chitchat, and Holmes's deductive powers come up. Mary shows some suspicion at Holmes's deduction skills. He points out several basic details about Watson simply from his walking stick, indicating that he is a decorated army veteran of the Afghan Wars. Mary asks Holmes to analyze her, over Watson's objections. He points out that she's a governess and has been engaged once before, but likely broke off the engagement because the fiancé wasn't wealthy enough for her. By this point Mary is starting to be come increasingly uncomfortable with Holmes's comments, but Holmes doesn't notice her discomfort or Watson's warnings. He just continues rambling on. He also adds that she's probably trying to do better this time around - e.g. finding a doctor to wed. Mary is extremely offended by his comment, and tosses a glass of wine in his face. She then tells him that everything he said was true, but her first fiance died. She then gets up to leave, thoroughly upset by Holmes's rudeness, and is followed by Watson. Holmes remains at the table and has his dinner, alone, though it looks like he expected this meeting to go bad.

Later that night, Holmes participates in a bare-knuckles boxing match. He's getting pummeled by the larger fighter, McMurdo, and seems to be losing. He's suddenly distracted by the appearance of a white handkerchief at the side of the ring with the monogrammed initials "IA." He scans the room and notices his old flame Irene Adler (Rachel McAdams), who then winks at him. Holmes makes to leave the ring - to the great disappointment of those cheering him on - but stops when his opponent spits on him.

Using the deductive logic we saw earlier, Holmes figures out how to use the handkerchief and some well-calculated moves to take out his opponent. We see the fight sequence twice - once as Holmes is planning through it, and then in real-time as he executes it: it starts when he flicks the handkerchief in front of McMurdo to distract him, then blocks McMurdo's blind jab and counters with a cross to the left cheek, and Holmes claps his hands over the opponent's ears. The dazed McMurdo tries to deploy a haymaker, but Holmes counters with an elbow block and one blow to the chest. Holmes then delivers a cross to the right side of the jaw that fractures the bone, delivers two more body blows and a right hook to the jaw, and finally a heel kick that sends McMurdo crashing out of the ring. He carries them out with blinding speed to the shock and surprise of everyone in the room. When the opponent hits the ground, the entire room falls silent in shock, and someone actually says, "Where did THAT come from?" Holmes looks around, and realizes that Irene's gone. He takes the money from a bookie and leaves.

Over at Scotland Yard, the prison guards are trying to deal with a potential riot. In front of Blackwood's cell, a guard is suffering a seizure, and no one wants to be around him for fear of his black magic. One of the braver guards asks him what he wants, and he says that he wants to talk to Holmes. It's his final request.

Meanwhile, we next see Watson walking up the stairs at the Punch Bowl tavern. He goes into a room, where he finds Holmes under the influence of cocaine, strumming away at a violin. Holmes greets Watson, and gestures to a bottle of flies that he has trapped. He tells Watson that when he plays a certain pitch, the flies will fly in a counterclockwise direction. Watson sighs and asks him how long it took Holmes to catch the flies, and Holmes admits about six hours or so. Watson raises his eyebrows and goes "Really? What if I do this...?" and opens up the lid, letting all of the flies escape. Watson then tells Holmes to get up and get decent, as Lord Blackwood has requested to see him.

Watson brings Holmes to the prison. On the carriage ride over, Holmes attempts to engage Watson in conversation, but Watson rebuffs him. Finally, Watson punches Holmes in the nose and tells Sherlock that he had already known about Mary's previous fiance. They bicker for a while, and finally arrive at the prison.

Most of the guards are afraid of getting too close to Lord Blackwood, so Holmes tells them that he can find his way on his own. Blackwood greets him, and tells Holmes that he is not done killing just yet. He warns Holmes that there will be three deaths that will be unpreventable. He also tells Holmes that trying to stop him will be an extremely futile gesture. Holmes ignores him, and makes several remarks about Blackwood's upcoming hanging and makes offhand comments about how even though Blackwood was cunning, his devious plans were going to come into an end. Finally Holmes leaves without paying much thought to Blackwood's warnings.

Blackwood is found guilty of murder, and then is hanged, and pronounced dead by Watson.

A few days later, Holmes is dozing in his study. Irene Adler, cracking walnuts, wakes Holmes up. She's brought him food from her travels. It is obvious the two have met before. Holmes looks rather unnerved and begins hastily straightening up the place; including slamming a portrait of Irene that he has on his desk facedown. The two of them banter for a bit, and it's implied that the two have some kind of romantic chemistry there. Holmes wryly comments on her con artist talents, and Irene just shrugs it off. She tells him that she needs him for a case, and hands him an envelope with information and money. She then gets up to leave, but not before putting Holmes's portrait of her into an upright position again. Holmes strums at his violin as she leaves.

Irene runs into Watson at the door, and greets him. Watson seems rather stunned, but doesn't say anything. We follow Irene down the street, until she gets into a carriage, where she meets with her shadowy employer. She tells him that she's sure Holmes will agree to take the case. Their conversation is interrupted by a street bum who suddenly runs into the side of the carriage. The carriage driver yells at the bum, who is scared away when the shadowy employer brandishes a wrist-mounted pistol.

At Baker Street, Watson chastises Holmes for falling for Irene. Holmes denies this, but Watson continues to tease him about this. Watson has read the file: a missing red-headed dwarf named Luke Reordan, who can be identified by his absent front teeth. He makes some jokes about Irene's taste in men, and asks Holmes what he was doing. Holmes starts to explain.

We cut back to Irene as she left Holmes's study, while Holmes pretends to strum at his violin. Once she closes the door, Holmes rushes over to the window just in time to see her walking down the street. He hurriedly comes up with a plan to follow her. Watson has just passed Irene at the door and has reached the midway landing when Holmes comes running downstairs in a hurry. Watson tries to ask Holmes what he is doing, but Holmes is obviously in a hurry, and he doesn't explain anything to Watson, and steals his coat. Holmes jumps out the window and crashes on top of the coal shed. Watson sighs and closes the window, ignoring Holmes.

Holmes, covered in soot, manages to catch up to Irene in another alleyway. As he hides, two muggers accost Irene. However, she turns the tables on them, and at knifepoint steals the first man's wallet and a bouquet of flowers. He follows her through a circus, picking up small little items on the way to pick up a disguise, careful to avoid being seen. Leaving the circus, he deliberately runs into the side of the carriage, and is scared off by Irene's employer with the wrist pistol.

Back in the present, Holmes finishes his story. As they're talking, a constable from Scotland Yard(William Houston) comes in and tells them that Lord Blackwood has apparently risen from the grave. Holmes wants Watson to come with him, but Watson says he needs to visit with Mary. Holmes obviously wants Watson to come with him, so he berates Watson for not caring about his reputation, saying that no girl wants to marry a man who can't tell if a man is dead or not. Watson finally agrees to go. They're taken to the cemetery to meet with Lestrade, where the limestone outside of Blackwood's mausoleum has shattered. Holmes examines the scene and licks a piece of the stone and waits as Lestrade and his men bring the coffin out. Once they open the coffin, they find a completely different body. His missing front teeth identify him as Luke Reordan, the missing ginger midget Irene had asked Holmes about.

Watson determines that Reordan has been dead for approximately twelve hours. Holmes takes Reordan's watch without anyone noticing, and then tells Lestrade he'll follow up. Holmes and Watson leave the cemetery.

Later, Holmes discusses the initials scratched on the watch. Holmes tests Watson's deducing skills by having him explain a pair of scratch marks - which Watson correctly deduces as indicating that the owner was a drunk. Watson deduces that the initials on the watch, MG, are pawnbrokers' marks, and quite by coincidence, they see the shop that sold the watch, and they get an address.

Outside the pawnbroker's place, Watson is approached by a gypsy woman who claims she knows about him and Mary. He tells the woman to tell him his fortune, and she tells him that marriage is a bad idea, as it'll consist of Mary growing fat and getting a beard, print wallpaper and lace doilies. Holmes solemnly echoes everything she says, and Watson realizes that Holmes has already paid off the gypsy to tell him a false fortune. He asks if Holmes has no shame, and then - just to spite Holmes - buys an engagement ring for Mary on the spot.

After Watson purchases the ring and Holmes has obtained Reordan's address, Holmes makes to go in and examine the building. Watson tells him he can't follow, as he has a dinner planned with Mary's parents. Holmes looks disappointed but understanding, and goes in alone. Watson stands in the street for a minute before sighing to himself, and follows.

Holmes is in the middle of picking the lock on Reordan's door when Watson just kicks the door in. As the two search the premises, they determine that Reordan sensed something was coming to get him. Holmes determines that Irene Adler has been by recently, as he smells her perfume. The room is in shambles, filled with medical equipment and experimented-on animals of various kinds, including dissected frogs and rats. Reordan also appears to have been working on some sort of attempt to combine sorcery and scientific formula together. Watson finds a partially burned piece of paper with Lord Blackwood's watermark - effectively linking Reordan to Blackwood - and Holmes snips the tail off of a experimented-on (and dead) rat. Holmes also observes a plate with some kind of melted substance and honeycombs, as well as some sort of gunpowder that burns with a pinkish hue when lit with a match.

Holmes is right in the middle of saying that everything fits in with what he's been thinking, but there was one smell he couldn't identify - toffee. It is at this point that two of Blackwood's henchmen come in, one of them eating a caramel apple (the scent that Holmes was smelling), and Watson calls Holmes's attention to them. Holmes notes that they're carrying canisters of lighter fluid in their hands, and asks the men if they're here to burn down the building. They smile sinisterly and say yes, and then call out for their colleague - an extremely large man named Dredger(Robert Maillet). Dredger, who only speaks subtitled French, asks if there is a problem. Holmes takes on Dredger while Watson takes on the two smaller thugs.

As they fight, Holmes pauses to catch his breath, and ask for a moment, and Dredger shrugs and politely says it's not a problem.

The fight comes to a head when Holmes discovers an electricity-powered rod - an early version of a taser - amongst Reordan's inventions. Holmes wields it at Dredger, and a slight touch throws Dredger backwards through a wall with the force of a shotgun blast. Dredger regains his footing, but another zap causes him to fly backwards and crush one of the thugs holding Watson hostage. Even though Holmes admits that he doesn't know how it works, he chases Dredger into the street.

Holmes chases Dredger down to a London shipyard. There's a half-finished boat in the yard, and the men all look up in interest as Holmes threatens to zap Dredger. He asks the man who he works for, and Dredger responds, "You know who", before reaching over and crushing the rod. Now defenseless, Holmes can only run.

The two men duke it out in the shipyard as Watson follows them in.Through some miscalculations on Watson's part, the moorings for the boat are released and the half-finished boat is prematurely launched into the Thames, where it sinks. Dredger flees the scene, and Watson - having dived to save Holmes from the moving boat - and Holmes are left looking at the sunken ship.

Watson and Holmes are arrested and subsequently held in an outdoor pen. It looks like a drying out tank for minor criminals. Watson complains to Holmes that all he wanted to do was eat dinner with Mary's parents, and it becomes a full-on rant about Holmes's various bad habits. Holmes tries to deflect Watson, and their argument really seems like that of an old-married couple.

Just as Watson's about to really get going, a police guard calls Watson's name and tells him Mary has posted his bail. Watson gets up and Holmes follows, but Holmes is told that bail has only been posted for Watson. As the gate slams behind Watson, one of the other criminals being held tells Holmes that he better get out soon, as the criminals are getting "hungry".

The next day, a large group of people are surrounding something. The guards are worried it might be a riot, but it turns out to be Holmes sitting down with the criminal from earlier, cracking jokes. The crowd laughs uproariously, before a guard steps in to tell Holmes that his bail has been posted. When Holmes asks who posted the bail, the guard just says that Holmes has friends in high places. We see a rich-looking carriage in the distance, where a hand beckons to Holmes. He steps in, and is told he has to be blindfolded.

When Holmes's blindfold is removed, he's inside a large mansion. Standing in front of him is Sir Thomas Rotheram (James Fox) and another associate. They apologize for blindfolding Holmes, saying they needed to protect their location. Holmes scoffs a little, and promptly gives a full turn-by-turn recap of the route they just took, simply based on observations using his other senses. He then notes the ring on Sir Thomas's finger, and concludes that he is in the headquarters of the Temple of the Four Orders in St. James's Square. It's at this point that the Home Secretary Lord Coward (Hans Matheson), and American Ambassador Standish steps into the room as well.

Sir Thomas, Lord Coward, and Standish explain that Temple of the Four Orders is a secret society similar to The Illuminati and the Freemasons who practice magic for good. They tell Holmes that Lord Blackwood was once a member, but they're having trouble controlling him. They want the situation resolved, and are willing to put everything they have at Holmes's disposal. Lord Coward tells Holmes that as he is the Home Secretary, he has control of the police force. Sir Thomas tells Holmes that the key to Lord Blackwood's power is in a book of spells. He hands a book to Holmes who flips at it disinterestedly. Sir Thomas says he knows that Holmes doesn't believe in magic, but it's important.

Holmes says he'll help, but not because he wants their resources. He's genuinely curious about the situation. He also reveals that Sir Thomas is Lord Blackwood's father, based on the fact that they have identical physical traits. Sir Thomas seems stunned, and says that very few people know of the relation. Sir Thomas conceived Lord Blackwood with one of the women used in their rituals, and Blackwood has been evil since his youth. Holmes says that given that everyone Blackwood is close to has been killed, Sir Thomas is probably Blackwood's next victim. The three men look at each other anxiously as Holmes leaves.

Irene Adler finds Holmes trying to get into her hotel room, and hands him a bottle of wine saying, "Here's something you might be able to open". As she changes behind a screen and with her back to Holmes, Holmes confronts her about Reordan's death. It's clear that she's very troubled by this news, but she plays it off for Holmes. Holmes urges her to flee, and she invites him to come along. He says he can't, but tells her that she has to go or he's going to put her in protective custody. Holmes confronts Irene about the man in the carriage with the wrist-mounted pistol, and remarks that he must be a professor - as evidenced by a chalk mark on his collar. Irene seems completely non-plussed that Holmes has followed her. We understand that this is a game of chase for both of them, and will always be that way.

Irene pours two glasses of wine and hands Holmes one. He gulps it down, and immediately begins to feel dizzy. He realizes then that the wine was drugged. Irene chidingly tells him that he shouldn't have refused to come along, and pulls him onto the bed. He passes out.

Late at night, as Sir Thomas is bathing at his house, he hears a noise, looks around but dismisses it. Suddenly, the water starts bubbling, the lights go out, and Lord Blackwood appears. Sir Thomas tries to turn the water off, but his hand can't reach the faucet. Blackwood pushes his own father's hand into the water, then steals his ring. Blackwood stays until Sir Thomas drowns.

The next morning, at Irene Adler's hotel room, a maid comes in and shrieks with shock at what she finds: Holmes has been handcuffed to the bed, and has a pillow covering his genitals. Irene has even left the "key to his release" under said pillow. We then cut to Holmes sitting in a carriage, retelling the story to Clarkie, the constable who had fetched him to Lord Blackwood's gravesite. Clarkie finds the whole thing extremely amusing, and says it's a good thing that the maid was offended enough to call the police, as it helped Homes out of the situation quicker.

They pull up to Sir Thomas's mansion, and they begin their examination of the body/murder site. Holmes asks why the water's been drained, and the constable says it was out of respect. Holmes sits down and sort of laughs at Scotland Yard's incompetence. He tells them to look for a bath powder while he examines the scene.

After the last constable leaves the room, Sherlock begins tapping on the walls, and finally finds a latch that allows him to open up a hidden door. Behind the door is a small space with magic tools/symbols/etc. Holmes rifles through the items and picks up two skulls and a book of spells, which he slips into his coat pocket. As the constables come back into the room, he asks them if they've found the powder. They say yes and he leaves.

Ambassador Standish (William Hope), the ambassador to the United States, arrives at a nondescript building that night. Standish is greeted by Lord Coward and other members of the Order, who inform him that Sir Thomas is dead. Coward then tells him that they're voting on a new leader, and has already appointed Lord Blackwood. Standish seems stunned, but Coward says that this is the natural order of things. Blackwood (according to Coward) is powerful enough to make Britain regain its past glory, including winning back to the US. Blackwood appears, and says that the US is currently embroiled in post-Civil War turmoil and is ripe for the taking. He then sits down in the leader's chair.

Standish says that he won't stand for this, and decides to shoot Blackwood instead. Blackwood suggests he not do so. Standish ignores it, and pulls the trigger, and instantly bursts into flames, which shocks the other members. Standish leaps out the window and falls to his death, while Coward urges the other members to drink to their new leader.

The following day, Watson is packing up his medical studio. Holmes sticks his head in and asks if he can use Watson's study, now that it's no longer occupied. Watson seems a little hurt but agrees, and Holmes carries in a body. It's from one of the henchmen who attacked Holmes and Watson at Reordan's apartment, who apparently died after his neck was crushed by Dredger's weight.

They examine the body, and try to figure out where the man was before his death. They determine that he was working out of a meat factory near the Thames, a lead Holmes wants to chase down immediately. Holmes tells Watson he doesn't have to come, but deliberately leaves his pistol on the table to get Watson to follow his lead.

That night, Holmes and Watson travel with Tanner, a boat captain who knows London's waterways better than anyone else. Tanner and Holmes are laughing uproariously while Watson's struggling with manning the boat. Watson complains that they're not doing anything, but Holmes and Tanner just laugh at him.

They dock the boat at the factory. Holmes and Watson go inside, and the setup is very similar to the redheaded dwarf's place. There's a lot of scientific equipment, and a biblical quotation that refers to the end. As Holmes and Watson stalk around, they notice a row of pigs hanging on a line, ready to be chopped into pieces. They're interrupted by Lord Blackwood's voice, who says that he warned Holmes all of this would happen. Holmes and Watson back up against the wall with their weapons brandished, but they can't figure out where the voice is coming from. Blackwood's face suddenly appears in a crack in the wall behind Holmes and Watson, and both men jump, turn around and shoot at the noise. However, Blackwood manages to get away, but not before 1) saying that "she" followed Holmes and Watson and 2) setting off flames. Indeed, he's kidnapped Irene Adler and tied her up to the pig line, surrounded by flames. Watson cautions Holmes that "this game is going to hurt".

Holmes wraps himself in a blanket and races towards Irene. He throws the blanket around Irene, trying to protect her from the flames. At the same time, Watson begins fiddling with equipment to get the flames to stop. He manages to stop the flames, but the line is still headed towards the blades.

Holmes sees a box of scraps, reaches up and throws the scraps in the gears of the line, trying to jam it. It works for a couple of seconds, but the scraps move and the line continues to move. Holmes tells Watson to prop Irene up, while he tries to figure out what to do. Watson races over, and Holmes grabs a saw and tries to hack away at the chains binding Irene to the line. It doesn't work.

They're only seconds away from the animal-slicing saw now, and Holmes sees a steam pipe. He tells Watson to turn it on, and he effectively overloads the pipe and brings the line crashing down. Irene is saved from the saw, only seconds away from being chopped up. Once freed, she hugs Watson and Holmes, also leaning to kiss Holmes on the cheek.

Watson goes after Lord Blackwood with Holmes following close behind. As Watson runs, he's too distracted by Blackwood's getaway boat fleeing the scene, and hits a trip wire. He turns around to see Holmes behind him, and yells at Holmes to stop. Unfortunately, the line has already been tripped, and a series of explosions rips through the building. Watson and Holmes are separated by blasts, so Holmes grabs Irene and tries to shield her to the best of ability. The explosions go on for some time, and Holmes finally blacks out.

When Holmes finally comes to, the constable from earlier (the one that summoned him to Blackwood's grave and escorted him to Sir Thomas's murder scene) is shaking him. He tells Holmes that Lord Coward has utilized his powers and issued a warrant for Holmes's arrest. He also informs Holmes that Watson's doing okay, but Holmes must flee immediately. Holmes gets up and begins to run.

We cut to Irene Adler dressed and about to board a train. She asks a conductor if the train is delayed, and he assures her it is not. She gets into her car, and we see the shadow figure from earlier sitting in a corner, reading a newspaper. He tells her that the train will leave when he wants it to; He also chides her saying that he hired her to manipulate Holmes's feelings for her, not to have her succumb to them. Irene gulps.

Cut to a doctor leaning over Watson in a hospital ward. Watson's asleep, and we can see that he's severely injured. We see Mary approach the bed and ask the doctor how Watson's doing. The doctor hastily excuses himself, and Mary follows, asking if that's "the best he can do". We get a close-up and see that the doctor is actually Holmes in disguise.

Holmes looks guilty and anguished, but Mary gently assures him - without ever acknowledging his identity - that Watson wouldn't have gotten involved if he didn't want to. He saw the whole thing as an adventure. She tells Holmes that everything's going to be okay, but he needs to fix it.

Cut to Holmes sitting in the opium room at the Punch Bowl from earlier, thinking over the situation. He pulls out the bones he collected from Sir Thomas's, along with the spell book. Holmes decides that the only way to truly understand the situation is to understand the magic, and attempts to create one of the spells. His mind wanders in a daze, and he passes out.

Cut to Holmes on the bed, waking up to Irene staring down at him. She greets him, and then we see that Watson is sitting in the room as well. Watson comes over to sit on the bed with him, and they have an awkward moment of not looking each other, grumbling that they're glad the other is okay. (It's very much another bromance moment.) Irene just studies them with amusement.

Watson asks about Holmes's recreation of the magic (there're really detailed pentacles and animals drawn all over the floor), and Holmes explains that it allowed him to understand Lord Blackwood's next move.

* He explains that based on the spell book, spell practitioners will always have a drawing of a pentacle and a cross in the pentacle. * He pulls out a map and says that using the Order's temple as a center point (the building where the US ambassador was killed), Blackwood's earlier five victims were all killed at the five points of the pentacle. * The following three victims (following Blackwood's resurrection) were killed at the points of the cross, and each death corresponded to an animal. (Each cross point has an animal). * There's only one cross point left, whose animal is a lion. It means that the next site is going to be the House of Parliament, as both the House of Commons and the House of Lords are meeting today.

At this point, Sherlock's explanation is interrupted by the arrival of Lestrade and his men. Sherlock tells Irene and Watson to escape through a trap door, and hands Watson an envelope with a list of instructions. Holmes remains to face off with Lestrade.

Lestrade takes Holmes in a carriage to the House of Parliament and delivers him to Lord Coward's office. Lestrade comes in, and apologizes for interrupting Lord Coward. Lestrade pulls open a coat and shows a pin with what appears to be the Order's symbol, and says that he has brought Holmes to Lord Coward to protect the order. Lestrade accuses Holmes of spreading rumors about the Order, while Holmes retorts that he now knows that being an Order member is how Lestrade managed to become an inspector at Scotland Yard.

Lestrade excuses himself, and Lord Coward begins taunting Holmes. Holmes manages to close the flue on Lord Coward's fireplace, before inquiring about Lord Coward's plans for the Order.

Coward (with his back to Holmes) admits that they only want true believers in the government, but tells Holmes he'll never figure out how Blackwood, Coward and the Order plan to take out the non-believers.

Holmes scoffs and tells Coward that he's already figured it out. He's noticed that Coward's boots were stained with mud when he walked in, and Coward smells like the sewer. Undoubtedly, Blackwood has set his trap up in the sewers beneath Parliament.

(We cut to a quick scene of Blackwood, Coward and followers anointing a site in the sewers.)

Holmes also adds that he knows Coward was with Blackwood from the beginning: Coward's shoes are one of a kind, and Holmes noticed them when he broke up the ritual three months ago.

Coward pulls out a weapon and turns around to shoot Holmes, but the closed flue has allowed the room to fill with smoke. He stalks around the room trying to sense Holmes out, but we see that Holmes is actually sitting behind him, smoking his pipe. Holmes tells him that they have a lot to get done, darts across the room, and leaps out the window.

Once Holmes hits the water, he holds up a pipe and is promptly thrown a rope by Watson, who is waiting in the boat from earlier. Holmes gets on, and is wrapped in a blanket by Irene.

They discuss what Holmes has figured out regarding the site being in the sewers. He also tells them that he had worked out with Lestrade on how to get into Lord Coward's in the first place. We cut to a flashback of Holmes telling Lestrade what to say/how to act on the carriage, including pretending to be a member of the order.

Following this, Holmes directs them to an entrance for the sewers.

Cut to both Houses of Parliament filing in. Coward's examining the scene with a knowing smile.

Cut to Watson, Holmes and Irene in the sewers. They're greeted with a device loaded with poison that is set to be released into the vents surrounding the House. Several men are guarding it, and Watson hurriedly confers with Holmes on what to do, but they're interrupted by Irene whipping out a pistol and shooting at the men. Watson remarks that Holmes's muse "certainly likes to make an entrance".

The three of them begin brawling with the guards, and they're only interrupted by the arrival of the French man from earlier. Irene attempts to shoot at the French man, but only upsets him. Holmes chides her, and tells her to figure out the device.

Upstairs, Lord Coward announces the return of Lord Blackwood to the disdain and disgust of most of the room. Blackwood says that they need to join together, and believers will revive England, etc. Lots of head shaking and disdain follow.

Back downstairs, Irene's having trouble disarming the device. She says that it's been designed in a way that any attempts to stop it will fail. Holmes and Irene agree that the device is somehow going to be triggered remotely. Watson continues fighting.

Upstairs, Lord Blackwood says that all non-believers will perish. Members of the Order make to stand at the doors and link arms, to prevent everyone else from leaving.

Downstairs, Holmes and Irene agree that the only way is to block the release of the device, pull out the canisters of poison in the device, and set off a remote explosion. Irene manages to pull the canisters out, and goes racing off into the sewers. Holmes helps Watson take out more guards, and then chases after Irene.

Upstairs, Lord Blackwood sets off the device, but nothing happens. Coward looks confused, and Blackwood stalks off to the sewer to find out what's going on.

Irene continues to race through the sewers, looking for a way out. Holmes follows behind. Blackwood notices a blur running past and follows as well.

Irene finally reaches open air, only to discover that she's somehow ended up on the scaffolding for a partially completed Tower Bridge. Holmes appears behind her, declares that he wasn't going to chase her anymore, and tells her to leave.

Irene looks exhausted, and says that she'll tell Holmes everything. He replies, "I wish you would". But before she can speak, she's interrupted by Blackwood, who pushes Irene off the scaffolding. She hits another scaffolding piece below, and is knocked out.

Holmes and Blackwood face off, and Holmes tells him that he figured out that all of Blackwood's "magic" is just well plotted scientific tricks.

* Lord Blackwood didn't resurrect from the dead - he had a clip on his noose that shifted the weight to his waist. * The limestone outside of Lord Blackwood's grave wasn't shattered by supernatural force. Instead, it was shattered beforehand, and glued together by the honeycomb substance Holmes and Watson saw while going through Luke Reordan's lab. (This explains a scene where Holmes is licking the limestone at the cemetery.) * The Ambassador wasn't killed by supernatural force. Instead, the rain showering him was a gas that triggered from rigged gunpowder fuel when the ambassador pulled the trigger. * Sir Thomas's bath powder, when mixed with the metal content from Thomas's ring, caused the water to boil. * The reason the Order members wouldn't have been poisoned is because Blackwood had them drink the antidote to the poison after the ambassador was killed. The antidote was disguised as holy water.

Blackwood congratulates him on figuring it out, and Holmes mentions that he can't wait to see Blackwood in jail. Blackwood reaches for a weapon to kill Holmes, but accidentally trips off a scaffolding and falls to his death. His body is left hanging off of Tower Bridge, which is supposed to be ironic, considering the plans for the original execution.

Holmes goes down to where Irene is, and waits for her to wake up. She tells him that her original employer is a man named Professor Moriarty, a man who is dangerous and just as smart as Holmes. She warns him that Moriarty is a very dangerous foe, but Holmes laughs it off. He then puts handcuffs on her and drops a key down her shirt, a call back to the hotel scene. He kisses her on the head, and then leaves her on the scaffolding.

Cut to Watson walking down the street with Mary, and Mary asking if Holmes has really reconciled himself to Watson moving out. Watson says he has, and gestures to the replacement ring Holmes bought for her - a huge diamond.

They go inside, and Mary seems taken aback to see Holmes hanging from the rafters. Watson is non-plussed though, and says that Holmes is too arrogant to kill himself. He pokes at Holmes, and Holmes awakens. Holmes explains that he was testing out how Blackwood prevented his death at the first hanging attempt - the clip distributing the weight to the waist. He asks Watson to cut him down; Watson refuses. Only after Holmes has explained everything Watson finally cuts Holmes down from the rafters.

Mary notices that the dog is unconscious again, and Holmes says the dog will be fine in a minute. It's at this moment that Holmes learns that a piece is missing from the machine Blackwood attempted to use against the House. He figures out that Moriarty took a piece while Holmes was busy chasing after Irene. Holmes figures out that Moriarty may be a more dangerous foe than he originally thought. Watson and Mary leave to go to their new home, and Holmes is left alone to muse over the new case that has been brought before him.