It's been one year since the Pevensies and Elaine have returned from Narnia. Since then, they have befriended Eva, who moved into a nearby house shortly after them. Eva adores all the Pevensies, but she also knows that they are hiding something from her. Peter and Elaine, who still consider themselves husband and wife, are facing tension in their marriage. While waiting at a train station to take them to boarding school, all of them, including Eva, are whisked back to Narnia. Their excitement soon turns to dread when they discover their old palace in ruins and later learn that 1300 years have passed for the Narnians, and the Telmarines have since invaded and taken over. Their only hope is to find the young Prince Caspian who is on the run from his evil uncle, Miraz, who wants him dead so he can take over the throne.
Meanwhile, Caspian's older sister, Cassia, remains in the castle as a firm supporter of her uncle and his ideology. However, she is thrown for a loop when Narnians she had believed to be extinct turn up, and she believes them responsible for her brother's disappearance.
Against Caspian's advice, Peter leads a raid on the castle to prevent a battle with the Telmarines. It is during this raid that Caspian and Cassia learn that Miraz murdered their father 18 years ago. The raid fails, many Narnians are killed, Cassia becomes a prisoner of war...and then Caspian and Eva nearly release an old enemy. Everyone must learn to stop trying to do everything on their own and remember who is really in charge in Narnia before the Telmarines wipe out the Narnians for good.