While standing on a British railway station, Peter, Susan, Edmund, and Lucy Pevensie are magically whisked away to a beach near an old and ruined castle. They determine the ruin is Cair Paravel, where they ruled as the Kings and Queens of Narnia, and discover the treasure vault where Peter's sword and shield, Susan's bow and arrows, and Lucy's bottle of magical cordial and dagger are stored. Susan's horn for summoning help is missing, as she left it in the woods the day they returned to England after their prior visit to Narnia. Although only a year has passed in England, 1300 years have passed in Narnia.[a]
The children intervene to rescue Trumpkin the dwarf from soldiers who have brought him to the ruins to drown him. Trumpkin tells the children Narnia's history since their disappearance: Telmarines have invaded Narnia, which is ruled by King Miraz and his wife Queen Prunaprismia, but the rightful king is Miraz's nephew, Prince Caspian, who has gained the support of the Old Narnians.
Miraz usurped the throne by killing his brother, Caspian's father King Caspian IX. Miraz tolerated Caspian as heir until his own son was born. Prince Caspian, until that point ignorant of his uncle's deeds, escaped from Miraz's Castle with the aid of his tutor Doctor Cornelius, who schooled him in the lore of Old Narnia, and gave him Queen Susan's horn. Caspian fled into the forest but was knocked unconscious when his horse bolted. He awoke in the den of a Talking Badger, Trufflehunter, and two dwarfs, Nikabrik and Trumpkin, who accepted Caspian as their king.
The badger and dwarves took Caspian to meet many creatures of Old Narnia. During a midnight council on Dancing Lawn, Doctor Cornelius arrived to warn them of the approach of King Miraz and his army; he urged them to flee to Aslan's How in the great woods near Cair Paravel. The Telmarines followed the Narnians to the How, and after several skirmishes the Narnians appeared close to defeat. At a second war council, they decided to wind Queen Susan's horn, in the hopes that it would bring help, whether Aslan or the fabled Kings and Queens of Narnia's Golden Age.
Trumpkin and the Pevensies make their way to Caspian. The trek proves difficult, but Aslan appears to Lucy and instructs her to guide the others behind him. Aslan sends Peter, Edmund, and Trumpkin ahead to Aslan's How to deal with treachery brewing there, and follows with Susan and Lucy.
Once Peter, Edmund and Trumpkin arrive, and drive out or kill the creatures threatening Caspian, Peter challenges Miraz to single combat; the army of the victor in this duel will be considered the victor in the war. Miraz accepts the challenge, goaded by Lords Glozelle and Sopespian. Miraz loses the combat, but Glozelle and Sopespian declare that the Narnians have cheated and stabbed the King in the back while he was down. They command the Telmarine army to attack, and in the commotion that follows, Glozelle stabs Miraz in the back. Aslan, accompanied by Lucy and Susan, summons the gods Bacchus and Silenus, and with their help brings the woods to life. The gods and awakened trees turn the tide of battle and send the Telmarines fleeing. Discovering themselves trapped at the Great River, where their bridge has been destroyed by Bacchus, the Telmarines surrender.
Aslan gives the Telmarines a choice of staying in Narnia under Caspian or returning to Earth, their original home. After one volunteer disappears through the magic door created by Aslan, the Pevensies go through to reassure the other Telmarines, though Peter and Susan reveal to Edmund and Lucy that they are too old to return to Narnia. The Pevensies find themselves back at the railway station.