One year after their previous adventure, the Pevensie siblings—Peter, Susan, Edmund, and Lucy—are whisked back to the magical land of Narnia through a subway station portal. They discover that more than a thousand years have passed in Narnian time since their last visit. Narnia is now ruled by the tyrannical King Miraz, who has usurped the throne from its rightful heir, Prince Caspian, his nephew. To ensure his own son's succession, Miraz seeks to eliminate Caspian. The young prince flees into the forest to escape assassination, where he blows Susan's magic horn to summon help, leading to the return of the Pevensie children.

The siblings find that Narnia has been conquered by the Telmarines, a race of humans who have suppressed Narnia's magical creatures, claiming them to be mere myth. As they traverse this altered landscape, the children reunite with Aslan's loyal subject, the Dwarf Trumpkin, and agree to aid Prince Caspian in gathering a Narnian army. This coalition includes Old Narnians—Talking Beasts, Dwarfs, Centaurs, Fauns, and Giants—who have been driven into hiding. Together, they aim to restore Narnia to its former glory, sparking a quest for freedom and justice. Their journey is fraught with challenges as they prepare to confront King Miraz and reclaim the throne for Prince Caspian.