On a cloudless night in Narnia, under an eclipse of its moon, inside a dark and forbidding castle, the Lady Protectress Prunaprismia (Alicia Borrachero) gives birth to a son. Down the hallway, General Glozelle (Pierfrancesco Favino) brings this happy news to Prunaprismia's husband, Lord Protector Miraz (Sergio Castellito). Miraz then tells Glozelle to follow earlier orders: to kill young Prince Caspian (Ben Barnes), the rightful King of Narnia, tenth in line from Caspian I ("The Conqueror"), who first led his Telmarine forces to invade and conquer Narnia, hundreds of years before.
But Caspian's loyal and devoted "professor," Doctor Cornelius (Vincent Grass), will not allow that to happen. He rouses Caspian from his sleep and hustles him into his armoire--the back of which opens to a secret corridor [an obvious reference to an armoire on a different world that opened into a radically different secret passageway]. Just as Caspian closes the armoire door behind him, Glozelle enters, with several men armed with crossbows, who then discharge their crossbows into Caspian's empty bed, and succeed only in filling the air with goose feathers. After which, Cornelius hurriedly assists Caspian to dress for a long ride, with his sword buckled on. The last thing that Cornelius gives Caspian is an ornate ivory horn, with the advice to blow it only at an hour of dire need--and that "everything you know is about to change."
Caspian escapes the palace and rides into the Shuddering Woods, with Glozelle and his summary-execution detail in hot pursuit. As Cornelius told him to expect, Caspian's pursuers do not care to follow him into the Shuddering Woods at night. Caspian then falls off his horse after colliding with an overhanging tree limb, and is set upon by two creatures, the nature of which he does not immediately recognize: dwarves, one redheaded (Peter Dinklage) and one black-haired (Warwick Davis). As they approach, the redhead sees Glozelle's detail, draws his sword, and rushes to meet them, telling his mate to "take care of him." As the black-haired dwarf approaches, Caspian grabs the horn and blows a long, loud note before the dwarf knocks him senseless.
But the peel of that horn carries across the dimensions--to 1942 London, where Susan Pevensie (Anna Popplewell) is trying to avoid a schoolboy who is trying to strike up a conversation, when her sister Lucy (Georgie Henley) summons her to the Strand Underground station. Inside, on the platform, Peter (William Moseley) has gotten into yet another pointless fight, and soon his brother Edmund (Skandar Keynes) leaps into the fray to come to Peter's aid, before two Home Guardsmen arrive, whistles blasting, roaring at everyone to "Break it up!" and "Act your age!" As the four sit down together on a platform bench, Peter complains bitterly about being treated like a kid--he who had been a High King as recently as a year before. Edmund reminds him that they are kids, and Susan has just told him that they ought to reconcile themselves to a life in England--when Lucy cries out, complaining that something has pinched her, and Peter and Edmund suspect each other of pinching one another. Suddenly Susan realizes that magic is at work. They all hold hands--and as an Underground train passes, the very walls of the Underground tube flake away and then dissolve, leaving the four standing, not in a tube train station, but in a shallow cavern near a sandy beach under a sunlit sky.
The children shuck off the outer coats of their school uniforms and play in the shallow water--and then Edmund wonders where they are. Peter is confident that they are back in Narnia, but Edmund does not recall that Narnia had ruins in it. He is looking up at the obvious ruins of a castle built high on a promontory overlooking the beach. The children climb the promontory and investigate the ruins--and finally they all realize that these are the ruins of Cair Paravel, where once they had reigned jointly after fighting a major battle.
Back at the Telmarine castle, Glozelle and his troops return from a sortie with a body draped over a horse. Miraz is eager to confirm the death of Caspian, but Glozelle stops him and shows him that he has brought back, not the body of Caspian, but a creature of an entirely different order: the red-headed dwarf who had challenged Glozelle, on foot and armed only with a sword (actually, more like a long knife for a human), in the Shuddering Woods.
An acrimonious council session begins with Lord Sopespian (Damian Alcazar) insisting that Miraz is not a man to be trusted. Miraz walks in, apologizing for being late, and then takes some pointed accusations from Lords Scytheley (Simon Andreu), Donnon (Predrag Bjelac), and Sopespian to the effect that Caspian is missing, on the very night that Miraz' son was born. Miraz answers with an easy scapegoat: Caspian, he says, has been abducted by Narnians. As "proof," he orders Glozelle to lead in the red dwarf, bound and gagged, and then charges that the Narnians, whom all Telmarines had supposed to be extinct, have been breeding steadily, waiting for an opportunity to attack the conquering Telmarines. Miraz then brazenly swears to find Caspian and "finish what our ancestors began" if he has to cut down every tree in the Shuddering Wood.
Back at Cair Paravel, Edmund detects the signs of an artillery barrage, the first sign that Narnia was in fact invaded, hundreds of years ago in Narnian time. The children investigate further, and find that the royal treasure room is still inviolate. With the aid of Edmund's new electric torch, the Pevensies find four trunks containing all their royal costumes and their original weapons and other accouterments--except that Susan's horn is missing. Lucy is a little sad as she realizes that every friend they had on Narnia is now long since dead. Peter buckles on his sword, takes up his silver shield with the red lion ramping up and facing the viewer, and announces that the four need to find out what has been happening on (and to) Narnia in what is now obviously an absence of centuries.
They don't have long to search. A small wooden rowboat, rowed by two Telmarine soldiers, comes down to the mouth of the Great River. Inside is the red dwarf, whom they are about to throw into the water when suddenly an arrow buries itself in their boat. An angry young woman (Susan) yells, "Drop him!" The soldiers throw the dwarf into the water--and as Peter and Edmund rush into the water to draw him out, Susan shoots one of the soldiers, causing the other to abandon their boat.
The dwarf at first isn't very grateful to them--until he realizes that he is in fact addressing the "Kings and Queens of old." By way of demonstration, Peter allows the dwarf, named Trumpkin, to fight a short bout with Edmund. Edmund takes a little while to get his sklls back but handily disarms Trumpkin, thus removing all doubt. "Maybe that horn worked after all!" he says.
Deep in the Shuddering Woods, Caspian wakes up and overhears the black-haired dwarf, named Nikabrik, and a talking badger named Trufflehunter (Ken Stott) discussing his fate. Caspian tries to fight Nikabrik, but Trufflehunter silences both of them and calmly asks Caspian to identify himself, which Caspian does. His story--that he is running away from his uncle who is probably nothing less than a usurper, impresses Trufflehunter, but does not impress Nikabrik nearly as much. Caspian prepares to leave, but Trufflehunter stops him and informs him of the true nature of the ivory horn he was carrying.
At the castle, Miraz angrily confronts Cornelius with the bright red arrow recovered from the body of one of his soldiers, and asks about Queen Susan's horn. Cornelius knows that his life is forfeit--but also that Caspian has done exactly as Cornelius expected. Aloud, he reveals that the Narnians believed that it could summon the Four Kings and Queens. Miraz, clearly not satisfied, orders Cornelius arrested--and Lord Sopespian, noticing, strikes up a frankly mutinous conversation with Glozelle. But Miraz doesn't give Sopespian long enough to dwell on such matters. He orders Glozelle to go to Beruna, taking a number of troops with him, to reinforce Sopespian's forces who have been building a bridge spanning the Great River at that point. "We must find Caspian before 'they' do," says Miraz, who then tells Sopespian that he needs a (classified) history lesson.
Trumpkin and the four children row up-river, as Trumpkin tells them that the trees never talk anymore, that "Aslan" apparently abandoned them with the four children seemingly did, and that many of the Talking Beasts of Narnia don't know how to talk anymore. (In fact, Trumpkin must kill a large bear that almost kills Lucy.)
In the Shuddering Woods, Caspian, Nikabrik and Trufflehunter set out to find other Narnians. Trufflehunter would much prefer that Caspian wait for the old Kings and Queens, but Caspian is in a hurry. Then Trufflehunter smells out a Telmarine search party, and the three must run for their lives. But before the Telmarines can kill them, they are killed themselves by a creature too small to see that obviously strikes from below, with deadly efficiency. Then this creature leaps out of the ferns that cover the ground in the forest, bowls Caspian over, draws a tiny sword, and shouts, "Choose your last words carefully, Telmarine!" This creature is Reepicheep (Eddie Izzard), who is actually a mouse! Reepicheep is inclined to kill Caspian out-of-hand, but Trufflehunter tells him to put away his weapon, because Caspian is the one who blew the horn--a revelation that gives Reepicheep pause. That is when a party of centaurs arrive, saying that the horn is why they have gathered.
At another part of the forest, Peter insists on leading the party along a pathway that Trumpkin insists is a dead end. In fact, he is looking for a crossing that no longer exists--because the River Rush has eroded for itself a canyon where a crossing once existed. Suddenly Lucy looks across the gorge, and spots Aslan, the leonine God-king of Narnia (Liam Neeson)! But no one will believe her, because no one but she can see Aslan. With one exception: Edmund is inclined to accept anything that Lucy says, because "the last time I didn't believe Lucy, I ended up looking pretty stupid."
That night, Caspian confronts a gathering of angry Narnians who debate whether to accept Caspian as a friend, or kill him out-of-hand. Reepicheep scathingly asks Nikabrik whether he remembers that his people once fought under the command of the ancient White Witch; Nikabrik says that he would do so again in preference to accepting Telmarine rule, or even an alliance. Trufflehunter scathingly tells Nikabrik that the other Narnians should be grateful that neither he nor anyone else can bring the White Witch back, and then asks whether he would have Caspian turn against Aslan. That sets off another round of angry catcalling. In the end, Caspian wins their favor by pointing out that he is a rightful king, and can bring about a lasting peace between Telmarines and Narnians. The lead centaur, Glenstorm, then tells the assembly that an astronomical conjunction has convinced him that the time is right for just such an alliance as Caspian offers. With Trufflehunter, the centaurs, and Reepicheep and his murine tribe to support him, Caspian gains the trust of the minotaurs and other Narnian creatures and is acknowledged as their leader.
The Pevensies and Trumpkin travel to the Ford of Beruna, but discover it occupied with a combat-engineering crew, led by Sopespian and Glozelle. They are building a bridge, cutting down many trees, and also building four huge trebuchets (a very intricate ballista, or boulder-throwing weapon). The implications are staggering--and Susan suggests that this wouldn't be a good place to cross after all.
The five then return to the spot where Lucy saw Aslan, and ask her to tell them *exactly* where she "thought [she] saw him." After protesting their patronizing attitude, Lucy abruptly falls through a layer of loose ground cover--to what turns out to be the beginning of a ledge that they can use to work their way down to the river and cross it.
That night, Susan awakens Lucy to ask her why she, Susan, couldn't see Aslan when Lucy can. Lucy doesn't know why only she can see Aslan. Susan reveals that she had finally gotten used to the idea of living in England, and the transition is actually difficult--mainly because their experience in Narnia will not last. The two girls fall asleep--but then Lucy, perhaps in a dream, sees a live dryad (which they have not seen in those woods since their return), and then sees Aslan, now standing much taller than he did before. Aslan tells her only, "Things never happen in the same way twice."
At that moment, someone stepping on a twig awakens Lucy, who finds herself back in camp with her brothers and sister. Then she sets out on her own, and thinks she sees Aslan again--but Peter silences her and shows her that she is seeing an armed minotaur. Peter gets closer to investigate--and another young man attacks him with a sword! Before they can harm one another, Lucy cries out to them to stop--because she realizes that this man is a friend, and is accompanied by a large group of unmistakeable Narnians. Surprisingly, they see centaurs, fauns, satyrs, minotaurs, badgers, and wolves--all on the same side, whereas 1300 years ago they had been enemies--and a new kind of Narnian: talking mice. (Lucy might or might not remember that a group of mice freed Aslan from his bonds at the Stone Table after he had submitted to a ritual sacrifice; in fact Aslan granted to mice the power of speech as a reward.) Then Peter realizes whom he's talking to: Caspian, the Telmarine prince-in-exile. Now Caspian, Peter, the other three Pevensies, Trufflehunter, Glenstorm, and Reepicheep (whom Lucy inadvertently offends when she calls him "cute"), have a round of mutual introductions and the beginnings of a war council. The united party, with many Narnians in tow, then sets out for their stronghold: Aslan's How, built atop the ancient Stone Table site. There Glenstorm's centaurian tribe greets Caspian and the four Pevensie children by erecting an arch of swords.
At the bridge site, Miraz interrogates Glozelle after an unknown raiding party (actually Reepicheep and his band of mice) have stolen enough weapons and armor for two regiments, and left this message: "You were right to fear the woods. X." (The X is for Caspian the Tenth.) Miraz then effectively demands that Glozelle pick a number of his men to have executed for sleeping on watch, or whatever charge would explain the losses in materiel; Glozelle says that he will prefer three for that punishment. Miraz then announces that Caspian has turned renegade--or, as we might say, gone native--and "Narnia is in need of a new king."
Caspian, Peter, and their party reach Aslan's How and take up residence there, passing under an arch of swords upheld by Glenstorm and his many sons and cousins. Susan and Lucy are most fascinated by another find deep in the How: the original Stone Table, still in the cracked-in-two condition in which they left it when Aslan was originally resurrected from the dead. There they see fresco portraits of the four of them, and of Aslan--but Peter, unimpressed, declares, "I think it's up to us now."
Peter knows that they have already been discovered--because a faun sentry has already reported seeing a Telmarine cavalry scout appear briefly at the edge of the woods and then ride off. Peter then outlines his plan: to attack the Telmarine castle, while the troops normally guarding it are out looking for them or preparing to move against Aslan's How. Caspian protests, saying that no one has ever successfully captured the castle, and that they ought to hold out where they are. Peter will have none of it. "This isn't a fortress; it's a tomb," he says--and could as easily say that one does not withstand siege without hope of a relief army, and that the best defense is offense. Lucy's is the only voice of reason: they ought to seek Aslan, rather than believing that the only alternatives are "dying here, or dying there."
Sadly, the massive raid on the castle ends badly. Caspian is able to rescue Doctor Cornelius, his old professor--but then wastes crucial time confronting Miraz and his wife over the death of his father. Miraz frankly admits his role in killing King Caspian IX, a revelation that embarrasses him in front of Prunaprismia--but that does not mitigate Caspian's failure to open the castle gate at a crucial time. Result: half of the invading force ends up killed, trapped and helpless and dying under a hail of crossbow bolts, while the other half can only watch helplessly before they must get away themselves. Glenstorm, in particular, sees at least half his sons killed in the action, and must break this terrible news to his wife when Peter and Caspian's battered forces return to the How.
Lucy alone does not participate in the invasion, and is available to administer her powerful medicinal cordial to the wounded (including Trumpkin, whose wound would have been fatal without it). But Peter and Caspian get into a bitter argument about failed plans, and whether Telmarines have any rightful place in Narnia at all. Edmund is the only person who can stop Peter and Caspian from killing each other in a duel at the very gate of Aslan's How.
As Miraz now has himself crowned King and accepts multiple troop pledges from the various feudal lords of Beruna, Tashbaan, Ettinsmoor, and the island of Galma, Nikabrik suggests to Caspian that he put his trust in another, more ancient power that once kept Aslan himself at bay. He introduces Caspian to a werewolf (Tim Hands) and a hag (Klara Issova), who inform him that they have between them reserves of endurance--and hate--that none can match, and can guarantee Miraz' death, and more. The hag then proceeds to draw a circle with her overgrown fingernail, and then draw out what looks like the broken-off half of a crystal sceptre, while chanting a harsh-sounding mantra. She then raises her voice and plants the sceptre on one edge of the circle. Whereupon an icy block forms in mid-air, and inside the block appears none other than Jadis (Tilda Swinton), the ancient and long-dead White Witch (and original owner of the sceptre, which once had the power to petrify any object--or creature--it touched before Edmund cleaved it in battle with the Witch). "One drop of Adam's blood, and you free me, and then I am yours, my king," says Jadis, with all the seductiveness she can muster. Nikabrik and the werewolf then lay hold of Caspian, and the hag slashes his left palm to force him to contribute the blood.
Suddenly Peter, Lucy, Edmund, and Trumpkin arrive and fight with Nikabrik, the werewolf, and the hag. Nikabrik almost kills Lucy before Trumpkin kills him. Edmund acquits himself best of all, by drawing the werewolf away and killing it with a single slash. Peter shoves Caspian aside and confronts Jadis--and that is a mistake, because Peter is now inside the circle. Jadis turns her charm on Peter, saying, "You know you can't do this alone." Before Peter can succumb to her charms himself, the icy monolith cracks, then shatters. Then Peter beholds Edmund, who has stabbed the block with his sword. Edmund waits a few seconds for Peter to thank him--a thing that Peter has never made a habit of doing--and finally says, "I know: you had it sorted." This is, of course, the second time that Edmund has rescued Peter from almost-certain death at the Witch's hands. (On the earlier occasion, of course, Jadis stabbed Edmund--who now has exacted revenge in kind.)
At last Peter realizes how mistaken he has been, and finally accepts Lucy's insistence that she can recognize Aslan and find him again. He complains that Aslan ought to have offered him proof, and Lucy suggests that perhaps Aslan was waiting for the Pevensies to prove themselves to him. Caspian, for his part, has a heart-to-heart talk with Cornelius, who tells him that he always looked for Caspian to be "that most noble contradiction: the Telmarine who saved Narnia."
Miraz' troops cross the now-completed bridge over the Great River, and arrive at the How--hundreds of them, with four trebuchets in tow. Now Peter announces his next plan: to send Susan and Lucy to ride deep into the Shuddering Wood to seek Aslan, because only Aslan can help them now. Caspian then advises Peter that Miraz has one more weakness: as king, he must satisfy the traditions and expectations of his people. One of them--deciding a battle by single combat--might buy them some time. Accordingly, Edmund, dressed in his original royal armor, walks boldly to Miraz' camp and delivers a written challenge from Peter to Miraz to engage him in single combat. Miraz' lords, using a little too much charm, suggest that Miraz is "within his rights" to refuse. Miraz is inclined to do anything but--because he will not back down in front of these men. This, of course, is exactly what Lord Sopespian wants--a fact that Miraz, on the occasion of the duel, clearly recognizes.
Susan and Lucy depart, astride Caspian's faithful war-charger. Caspian offers to give Susan back her ivory horn, but Susan declines, saying, "You might need to call me again"--a declaration that causes Lucy to ask her sarcastically what she means by that. Susan has neither time nor inclination to explain such a thing as romance to a little sister still too young to understand it.
As Peter and Miraz engage one another among some ruins outside the How, Susan and Lucy ride part-way into the woods, and then Susan sends Lucy on alone, and turns to meet a Telmarine patrol with her bow and arrows. Susan kills three Telmarines and risks dying anyway when Caspian shows up at the last minute, rescues her, and brings her back to the How.
Peter and Miraz manage to bloody one another, and then Peter asks for five minutes' respite, of which Miraz grants three. Peter and Edmund have a chance to make up for lost time, and Susan assures them that Lucy got through, with a little help from Caspian. Miraz, for his part, is irritated with Sopespian for not intervening earlier in the fight--a highly illegal thing that Miraz had asked Sopespian to do, but which Sopespian is disinclined to do, though not for any honorable reason.
The two leaders clash once again, and Peter finally defeats Miraz. Instead of killing him outright, Peter hands his sword over to Caspian, saying that Miraz' life is not his to take. Caspian is willing to spare Miraz, but will hand over the sovereignty back to the Narnians.
But Sopespian kills Miraz, using one of Susan's red arrows (which he probably stole from Dr. Cornelius' library after Miraz had rammed it into a drawing of the Ancient Kings and Queens), and cries treachery. This causes the Telmarines to launch a full-scale assault on the How.
Glozelle orders a cavalry charge while the artillerymen start a continuous barrage with their mechanized trebuchets. The Narnian forces manage to delay the Telmarines by undermining the ground under their horse's hooves, and then having a contingent of archers (under Susan's command) shoot a hail of arrows on the enemy cavalry after they fall into the sinkhole they have created, while Caspian and Glenstorm lead a cavalry charge into the Telmarines' rear. Then Sopespian sends in his infantry, and the defeat of the Narnians seems inevitable, especially after the artillerymen send their boulders crashing into the gate of the How, sealing it. But Lucy is able to get into the woods close enough for Aslan to meet Lucy--and pounce on another pursuing Telmarine patrol. "And now I think your friends have slept long enough. Don't you?" Aslan asks--and with a roar, he awakens the trees of Narnia from their thirteen-hundred-year slumber.
And so, at the last instant, Caspian has fallen into the sinkhole, where Glozelle, halberd in hand, is about to impale him--and hesitates. Before he can decide which side he really wants to be on, an animated tree root thrusts itself out of the ground behind him, picks him up, and pulls him into an overhang, knocking him senseless. As Peter helps Caspian back onto ground level, the trees, fully animated, walk onto the field by their roots and turn the tide of battle. One rock cast by a trebuchet knocks down one tree, which collapses and dies--but then another tree extends a long root to topple the trebuchet and crush it. With that, Peter, Caspian, Susan, and Edmund lead a countercharge, and the Telmarines fall back.
Sopespian withdraws his troops to the bridge, thinking that that might prove a more defensible position. Peter, Caspian, Edmund, and Susan pursue them, of course. Then, at the bridge, Sopespian is surprised to find himself confronted, first by a little girl (Lucy) armed with nothing more than a small dagger, and then by a tall lion, which ambles up to the bridge as though he owns the world. Sopespian orders a charge--and then Aslan, taking the measure of his enemy for about five seconds, lets out another roar. This time he awakens the god-like personification of the river, which nearly drowns Sopespian's troops--and does drown Sopespian himself, after lifting the bridge off its moorings, with Sopespian and his horse still standing on it.
After that, the Telmarine troops surrender their weapons to the Narnians. Aslan and Lucy receive Peter, Susan, Edmund, and Caspian. "Rise, Kings and Queens of Narnia--all of you," says Aslan, making clear to a suddenly diffident Caspian that he will well and truly be the next king of Narnia, on Aslan's direct authority.
Then the little band of mice bring Reepicheep forward on a litter. Lucy revives Reepicheep with her medicinal cordial, and Reepicheep bows before Aslan--before noticing that he has lost his tail. Aslan teases him about being overly concerned about his honor--but restores the tail anyway, after the other mice show their willingness to cut off theirs to spare their chief any humiliation. Finally Aslan asks Trumpkin to step forward, and roars at him as if to demonstrate that Aslan does in fact exist and is definitely someone to reckon with.
Aslan leads Caspian and the four Pevensie children through the Telmarines' capital city, whose inhabitants greet them all with cheers and garlands, and then into the castle, which from now on will have a far less gloomy aspect. Subsequently, Caspian calls an assembly of all the Telmarines, and announces that from now on, Narnia will be a Narnian-Telmarine federation, and that any Telmarines not willing to accept that may return to "the land of our forefathers." But this is not the land of Telmar to the west of Narnia. Instead it is an island on our familiar earth, where the Telmarines' ancestors, who were a crew of pirates, were shipwrecked on an island holding a portal to the Narnian world.
Glozelle, who has recovered from being dazed at the battle, offers to go first, followed by the dowager queen Prunaprismia and her newborn son. When they appear to disappear through the portal that Aslan creates, the other Telmarines express skepticism, and Reepicheep offers to lead eleven mice through the portal to serve as an example. That is when Peter announces that he and his siblings will go through the portal, because Caspian must now take charge. Furthermore, Peter and Susan will not return to Narnia again, though Edmund and Lucy will. Susan says good-bye to Caspian, saying that a romance between them would never have worked, because "I am about thirteen hundred years older than you are." And so the four step through the portal--and find themselves back on the railway platform of the Strand Underground station, where they board the next train. The only one to offer a regret is Edmund, who realizes that he has left his electric torch behind in Narnia.