The book starts with the Pevensie children, Lucy, Peter, Susan and Edmund, in England standing at a train station. They are suddenly transported to Narnia where they eventually find the ruins of Cair Paravel, their home for many years when they ruled Narnia. They discover their old artifacts, such as Peter's sword, Susan’s bow, and Lucy’s cordial. However, Susan's horn is not present, as she had left it in the forest when they had returned to England. They determine that despite only leaving Narnia a year ago, in Narnian years, this is equivalent to about 1400 years.
They then witness soldiers attempting to drown a dwarf by the name of Trumpkin. After saving him, Trumpkin reveals that a new race of men now rule Narnia, known as the Telemarines. Their presence has led to the talking beasts retreating to the woods and losing their ability to communicate with man. The ruler of the Telemarines is known as King Miraz, but his nephew, Prince Caspian is the true king.
Trumpkin reveals that King Miraz had killed his brother King Caspian IX in order to take the throne for himself. Prince Caspian had no knowledge of the fact that Miraz only kept him around because he had no personal heir himself. But, Miraz' wife had recently had a son, and now he no longer needs Prince Caspian as his heir. Prince Caspian's tutor, Doctor Cornelius, told him of this and gave him Queen Susan's horn should he ever need to call for help. He managed to escape the Telemarines but was taken unconscious after slamming into a tree branch by Trumpkin and Nikabrik, two dwarves who believed him to be their true King.
The dwarves and Prince Caspian met with the other creatures of Narnia in the woods, but later Doctor Cornelius found them and warned them that Miraz had sent the Telemarines after them. They all escape to Aslan's How near Cair Paravel. The Telemarines and Narnians engaged in warfare and the Narnians deliberated whether to blow Susan's horn. They sent Trumpkin to Cair Paravel, where he was captured by Telemarines. This is when he met the Pevensies, because the horn was blown indeed.
Trumpkin and the Pevensies travel to Cair Paravel and, on the way, Lucy spots Aslan. She determines that Aslan was trying to show them an easier path, but they ignore her and continue on their more dangerous and difficult path. However, in the night, Aslan comes to Lucy and informs her that they must follow his path, which they all do. They then see Aslan in the flesh. Aslan tells Trumpkin, Edmund and Peter to continue on to Aslan's How, where the fighting is issuing. They arrive there, but witness Nikabrik and his accomplices trying to summon Jadis, the White Witch, to help them win the battle as this would be the lesser of two evils. However, they are killed before they can do so.
Peter and Miraz engage in a single combat, which Miraz loses. However, his allies, Lord Glozelle and Lord Sopespian falsely accuse Peter of cheating. They continue the fighting, and it is Glozelle who then stabs Miraz in the back. Aslan, Lucy and Susan travel to awaken the Gods Bacchus and Silenus to help the Narnians in the war effort. They ensure that the Narnians win the battle and the Telemarines surrender in defeat. Aslan offers the Telemarines a chance to stay under Caspian's rule in Narnia or return to Earth where they originated from. The Pevensies go through the portal to assure the Telemarines and return to the railway station where the story began.