As Peter, Susan, Edmund, and Lucy sit at a rural train station, waiting for the trains that will take them off to their boarding schools (one for the boys, one for the girls), they find themselves transported to another world. They arrive on a thickly wooded island in the center of which sits a ruined castle. They eventually realize it’s Cair Paravel, where they themselves lived when they ruled Narnia. Although they only left a year ago in Earth time, that means thousands of years have passed in Narnia. They arm and armor themselves with items from the castle’s treasure room.
The next day, as they try to leave the island, they intercept two soldiers in a rowboat carrying out the execution of a Dwarf. Susan scares the soldiers off by shooting them from the trees (non-fatally) with her bow and arrows. She and Peter snag the boat and the children unbind the Dwarf (later identified as Trumpkin). After breakfast, Trumpkin tells them the story of King Caspian the Tenth, rightful ruler of Narnia.
Prince Caspian grew up in the court of his uncle and aunt, King Miraz and Queen Prunaprismia, hearing stories of the Golden Age of Narnia from his beloved Nurse. When Miraz fires the nurse for telling these stories, he gets a tutor called Doctor Cornelius. Cornelius, who is half Dwarf, half Telmarine (human) continues to teach Caspian about the old days in secret. When the king and queen finally have a son of their own, Caspian becomes superfluous to Miraz’s plans and Doctor Cornelius helps him escape the castle before Miraz can kill him.
As Caspian rides south toward safety in a neighboring kingdom, his horse throws him off in a thunderstorm. Two Dwarves (Trumpkin and another called Nikabrik) and a talking badger named Trufflehunter rescue the boy. Once they know who he is, they introduce him to other Old Narnians—talking beasts like Pattertwig the squirrel and Reepicheep the mouse, and magical creatures like Glenstorm the Centaur. Glenstorm, a prophet, tells them it’s time for the Old Narnians to revolt against Miraz under Caspian’s command. They form an army and take the ancient and magical place called Aslan’s How for their base.
When the war goes badly, Caspian blows a magic horn which once belonged to Queen Susan. This is what calls the children back into Narnia. Trumpkin was on his way to see if anyone had arrived at Cair Paravel when Miraz’s supporters captured him. Then the children rescued him, bringing his story back to the present.
Trumpkin and the children decide they must travel to Aslan’s How through the thick coastal woods. After a day rowing around the coast and another pushing through the forest, they reach the Rush River, which runs along the bottom of a nearly impassable gorge. Aslan shows Lucy the shortest path across, but her siblings doubt her and insist on going the longer way downstream. When an attack by Telmarine forces turns them back, they grudgingly follow Lucy, who follows Aslan. Aslan takes them directly to the How, restoring their faith.
As Trumpkin, Peter, and Edmund arrive, Caspian puts down a challenge from Nikabrik, who wants to turn to black magic to win the war. In the process, Nikabrik is killed and Caspian is wounded. Thus, when Peter suggests that the Narnians challenge Miraz to single combat to decide the war, it’s Peter who must fight. Goaded by his own lords Glozelle and Sopespian, Miraz accepts. After a long and evenly matched bout, Miraz trips and Glozelle jumps into the ring to stab him in the back. Telmarine and Old Narnian forces spring into open battle, which the Narnians win with the help of the reawakened dryads.
Meanwhile, Aslan, Susan, and Lucy roam the countryside, accompanied by Bacchus. They destroy bridges and set river gods free. Most Telmarines flee them in terror, but some—the ones who will fit well in the new Narnia—follow them. They return just as the final battle for Narnia ends, with Caspian and the Narnians emerging victorious. The Old Narnian forces dance, feast, and celebrate late into the night.
A few days later, Aslan summons the Telmarines who want to leave Narnia, which will once again belong first to the animals and magical creatures. He offers to send them through a magical door back to their ancestors’ world—Earth, as it turns out. It’s also time for the children to leave, and they lead the human exodus from Narnia, finding themselves back on the train platform just an instant after they left.