Edmund and Lucy Pevensie are staying with their cousin, Eustace Scrubb at his home, when they are all three suddenly transported to Narnia through a painting of a ship at sea. They land in the Narnian sea next to the boat in the painting, the Dawn Treader. The captain of the ship is determined to be King Caspian X, previously Prince Caspian. He is on a quest to discover the seven lost lords of Narnia. They also meet Reepcheep, a talking mouse, who longs to see Aslan's country.
They travel to the Lon Islands, where they meet the first lord, Lord Bern, who has participated in the slave trade in the land after being banished by Miraz. Caspian manages to take back the land for the Narnian's and installs Lord Bern as ruler of the land.
The next Island they visit, Eustace escapes in order to avoid helping them. He finds a dragon's lair and becomes greedy at all the wealth so hoards some gold. He falls asleep and when he awakens, he discovers that he has transformed into the dragon. Aslan helps to transform him back into a boy and as a result Eustance became a lot more aware and kinder to his cousins and the crew. Caspian determines that the Lord Octesian must have also been transformed into a dragon and eventually died.
They go to Deathwater Island where they see a pool that converts everything to gold. They determine that Lord restimar has also been turned to gold. They then travel to the Island Where Dreams Come True, which is an island immersed in darkness, and discover Lord Rhoop. They also travel to Island of the Star where three of the Lords are lost in an enchanted sleep. A fallen star on the island, named Ramandu, informs them that the only way to awaken them from their sleep is to sail to the edge of the world and sacrifice a member of their crew. They travel to the edge of the world and only Lucy, Edmund, Eustace, and Reepicheep set sail on a small boat to travel further. Reepcheep gives himself up and travels to the edge, where it is assumed that he reaches Aslan's Country - he is not seen again.
Edmund, Eustace, and Lucy then see Aslan, who tells Edmund and Lucy that they are too old to return to Narnia. They are all sent back to England.
