1,300 Narnian years after the events of the previous movie, Caspian, a Telmarine prince and heir to the Telmarine throne, is informed by mentor Doctor Cornelius that his uncle, Miraz, is plotting to kill him and seize the throne for himself and his newborn heir. Cornelius gives him Queen Susan’s ancient magical horn, instructing him to blow it only at his greatest need. Caspian flees into the woods where he encounters two Narnian dwarfs, Trumpkin and Nikabrik, and a talking badger named Trufflehunter. Being chased by Telmarine guards, and feeling threatened by Narnians, Caspian blows Susan’s horn.
In England (where only one year has passed), the Pevensie children are waiting for a tube train when the station suddenly starts collapsing, and they are magically transported to Narnia. At first they are overjoyed, but soon realize that much time has passed since their last visit, and their castle at Cair Paravel is in ruin with evidence it was attacked by catapults. In the Telmarine castle, the lords of the council learn that Prince Caspian is gone, and Miraz, using Trumpkin, who has been captured, persuades them to attack the Narnians to retrieve Caspian while secretly plotting to kill him instead. Lord Sopespian suspects Miraz’s motives, and discusses his treasonous thoughts with his ally, General Glozelle.
The Pevensies save Trumpkin from being drowned by two Telmarines; he realizes that the four children are the Kings and Queens of Old and they continue on together. Lucy glimpses Aslan and tries convincing the others that she has seen him; only Edmund believes her. Nikabrik and Trufflehunter lead Caspian to a gathering of the old Narnians, where he convinces them to help him win his throne so he can return their land. They meet the Pevensies and Trumpkin, and all journey to Aslan’s How. Peter decides they will attack Miraz’s castle, rejecting Lucy’s advice that they wait for Aslan to return. The Narnians raid Miraz’s castle, but Caspian ruins the plan by freeing Cornelius instead of opening the gate. He learns that Miraz killed his father, and confronts Miraz but is wounded. Overwhelmed, Peter calls for a retreat. Peter, Susan, Edmund, Caspian and the Narnians escape with heavy losses. Upon returning to Aslan’s How, Peter and Caspian argue, while back at the castle, Miraz is crowned King.