Three Narnian years after the events of Prince Caspian, Lucy and Edmund Pevensie are staying with their irritating bookworm cousin Eustace Scrubb until the war is over. Edmund is still too young to enlist in His Majesty's Armed Forces. At their cousin's home a painting of a ship on the ocean transports Lucy, Edmund and Eustace into an ocean in Narnia.
They are rescued by Caspian, who captains the Dawn Treader. He invites them on a voyage to rescue the seven Lords of Narnia whom his uncle Miraz banished. In the Lone Islands, where people are sold as slaves, Caspian and Edmund are captured and imprisoned while Lucy and Eustace are sold as slaves. Caspian meets one of the lost lords (Lord Bern), who reveals that the slaves are not sold, but sacrificed to a mysterious green mist. They are rescued by their crew. Bern, who becomes the new governor, gives Caspian a sword, one of seven given to each of the lords by Aslan.