After their first adventure in Narnia in The Lion, the Witch and the Wardrobe, the Pevensie siblings — Peter, Susan, Edmund, and Lucy — are living back in England. Although only about a year has passed for them in England, 1,300 years have passed in Narnia.
One day, while waiting at a train station (or preparing to return to school), they are magically transported back to Narnia. They arrive to find Narnia changed: the once-ruling magical creatures, talking animals, and forest spirits are oppressed. The land is under the control of the Telmarines, a human race that invaded Narnia long ago. The old Narnia—rivers, castles, forests—is in disrepair; Cair Paravel is in ruins.
The rightful heir to the throne, Prince Caspian, has been forced into hiding by his uncle, King Miraz, who seized power by murdering Caspian’s father. Miraz is threatened by Caspian’s existence, especially as Caspian grows older and more supported by old Narnians, those who remember the legends and the rule of good.
Caspian blows Susan’s ancient horn (a magical horn once owned by the Pevensies) in desperation, hoping the legendary children will return to aid him. That summons Peter, Susan, Edmund, and Lucy back to Narnia.
The Pevensies, along with Caspian and several Narnian allies — talking beasts, dwarfs, and loyal creatures — journey first to regain support, then into confrontation with Miraz's forces. Major landmarks on the quest are Aslan’s How (a mysterious hill/sealed chamber related to prophecy) and the gathering of Old Narnians.
A showdown is set: Miraz and his Telmarine lords challenge Caspian. Peter agrees to duel Miraz as a way to decide the battle. Although Miraz is defeated in combat, the Telmarines use treachery to ignite a larger battle. During the conflict, Aslan returns in power, awakening the trees and spirits of Narnia, helping turn the tide.
Ultimately, Caspian is restored as King of Narnia, peace is negotiated between Telmarines who wish to stay under his just rule and the Old Narnians. However, the Pevensies are told (by Aslan) that they are getting too old to return to Narnia, and when they complete their mission, they return to England — their adventure in Narnia concluding for them, though Narnia continues under King Caspian.