A man connected to the classification of a valuable building as a landmark trust dies when his pacemaker is deliberately short circuited. Holmes soon learns the one behind the murder is an old nemesis known to him only as "Moriarty".
- We open on a man returning home to find another man perched on his couch with a laptop. (We will learn later this man's name is Daniel Gottlieb.) With the laptop Gottlieb has the power to hack the pacemaker in the man's chest. He wants the man to "revoke" his vote on a committee he sits on. The man grabs his iPad, revokes his vote and... Gottlieb kills him anyway.
Sebastian Moran summons Sherlock to talk to him in prison and he gives him the dead man's obituary. He says Moriarty did it and Sherlock should follow up. He knows this because he was contracted to kill this man himself before Sherlock landed him in the clink. Moran then kills one of his prison guards, apparently for sport.
Sherlock lies to Gregson and says Moran told him nothing so he can investigate the claims on his own first. He and Watson go and perform an autopsy on the dead man and discover that his blood was literally boiled and he had marks on his hand consistent with being shocked and deduce that someone hacked his pacemaker.
After some other digging the discover the "revoke" vote he made right before he died. It appears his vote was the deciding one to knock an old prohibition era speakeasy off a historic register list meaning that a real estate/contractor could renovate the site now. All of the others who voted to revoke had contracting work done by the same man so they figure he must be behind it.
Until that man too is killed by an air conditioner falling out of a window. We see that that AC was pushed onto him by Daniel Gottlieb in an act of perfect timing.
After a bit of more digging they realize that a woman who was a vocal opponent to preserving the speakeasy status as a historic site will be the next target. They begin following her, basically using her as bait. They see she's wearing a medic alert bracelet that shows she's allergic to bees. They also discover that in the park that she runs through every single day now has a hive of rare Africanized honey bees, that if they stung her, would kill her almost instantly. Like the "natural" heart attack and the air conditioner "accident" it would look like the woman died of "natural" causes. Watson calls it a "baroque" plan but Sherlock admires it as quite tidy. They figure the man funding the bees will have to come back soon so they stakeout the beehive.
Watson wants to know what the plan is if they catch this man who works for Moriarty. Sherlock promises that he just wants to know who killed Irene and when they catch the guy he won't kill him. Watson doesn't believe him since nothing has changed since he almost killed Moran. Sherlock says he only made a real connection with one person, Irene. And now, in a roundabout way that makes it seem like he's talking solely about their teacher/student relationship, he says he's made another connection with her, which has changed him, or at least makes him want to see how his "project" turns out. She says it's one of the nicest things anyone has ever said to her. He says he'd like to promise her that he won't hurt the person responsible for Irene's death, just as he would that he'll never do drugs again but he's an addict and can't make promises but he asks her to trust his intent.
The "beekeeper" arrives it is Gottlieb. Sherlock claims they need to look at his notebook and then he tazes him.
They bring him back to the brownstone, tie him up in Sherlock's straitjacket and question him about the murders and Moriarty. Gottlieb has an encrypted message on his phone from Moriarty. They give him two options. Sherlock is going to send a message back saying things have gone wrong and they need to meet face to face and if by the time they get a response Gottlieb has told them everything he knows about Moriarty Sherlock will impose upon the police to give him special treatment. Or they'll just hand him over to the cops.
Gottlieb says he has killed 31 people, unclear if they were all for Moriarty, but doesn't know who he is. He does tell them what he knows: 8 years ago he was doing an environmental impact study in the U.K. and a tall British guy sat down next to him and asked him to dinner since he wanted to pick the brain of a serial killer who was passing in society. He had figured Gottlieb out and said he'd be in touch. He says it was all on the phone after that. He says he mentioned he spent time in reform school as a kid. He also says he knows Sherlock, and had an "accidental overdose" lined up for him but then another text canceled the job. He says it was the only time he got a cancellation.
After receiving a return message for a meet-up, Sherlock and Watson stake out the diner where it's supposed to go down. They don't expect Moriarty himself to show. Gottlieb, of course, doesn't show and they follow the man who looks like he was waiting for someone and takes off in a fancy car. He crosses some train tracks just as a train is coming effectively cutting off Sherlock and Watson. He meets someone on the other side. By using a superfast setting on his camera and opening the aperture wide Sherlock takes a series of fast photos, between the train trucks, slices up the images and pastes them together coming up with a man's face. Gottlieb confirms that is the man who recruited him.
Sherlock studies the photo. He worked his contacts in the U.K. and narrowed down those who did time in reform schools and prisons and found him. John Douglas. Turns out he was also brilliant and lethal as a boy. While in reform school his best friend was beaten to death by older boys and a few months later a few of his assailants were dead. He was released on his 18th birthday and he has no rap sheet. Sherlock thinks he's been using the Moriarty alias for 45 years. Watson wonders what they'll do next. Sherlock says he doesn't know.
The next morning Watson awakes to an apology from Sherlock. He says he knew more than he let on and he's sorry for abandoning her at the climax of their drama and he's of to see what he'd do with a loaded syringe.
He confronts Douglas in his hotel-- registered under his childhood friend's name-- and he's impressed he found him. He asks if he's Moriarty. He says he's not him, he could never be him, but he does know why he's looking for him: Irene. He says he didn't kill her but knows why she had to die: it was because of Holmes. And then Douglas gets shot in the back, right through the window.
The police arrive and Gregson wants to know what the hell is going on with Sherlock. He half-admits the stuff about Moran and this guy working for someone else and Sherlock is looking for him. He gives his word he won't repeat his Moran tactics. Gregson says he wishes his word meant more to him. Watson arrives and looks in her medical bag that Sherlock brought to intimidate Douglas. It's full of Lego. Sherlock offers this as proof that he's a changed man.
Watching the interrogation on the opposite side of the glass, Gottlieb is confessing, 18 murders so far says Gregson to Watson and Holmes. He has no interest in an attorney, he just wants to talk. He gets a text and they have him read it. Gottlieb can't translate it. Holmes brings it to Moran. He can't read it either, agreeing with Gottlieb that it's not a code they used. Moran does seem to know though and all he says is, "leave it" in a very menacing fashion.
Holmes works on decrypting the code. Watson offhandedly notes the time and Holmes realizes that is the decryption key: when the message was sent.
As he works it out we cut to Moran in his cell singing on old football drinking song. Sherlock cracks the code: it's a note to Moran. "You never told me you had a sister, either you die or she does- M." We cut to Moran bashing his head against the wall.
Sherlock tries to call Gregson to have him warn the warden but it's too late. Moran bashed himself into a coma and has terrible brain swelling. They realize Moriarty played him into bringing the text to Moran.
Sherlock says Moriarty had him in his sights twice and left him alive so he doesn't know what the game is. Watson points out that he may be the game. Gottlieb's phone rings. It's Moriarty, he says they're overdue for a chat.