The 3rd C. S. Lewis’s Chronicles of Narnia book is:
The two youngest Pevensie children, Lucy and Edmund, are staying with their odious cousin Eustace Scrubb while their older brother, Peter, is studying for an exam with Professor Kirke, and their older sister, Susan, is travelling through America with their parents. Edmund, Lucy, and Eustace are drawn into the Narnian world through a picture of a ship at sea. (The painting, hanging neglected in the guest bedroom in which Lucy was staying, had been an unwanted present to Eustace’s parents.) The three children land in the ocean near the pictured vessel, the titular Dawn Treader, and are taken aboard.
The Dawn Treader is the ship of Caspian X, King of Narnia, whom Edmund and Lucy (along with Peter and Susan) helped gain the throne of Narnia in Prince Caspian. Also present on board are the Lord Drinian (the captain of the Dawn Treader) and the first mate Rhince.
Peace has been established in the three years since then, and Caspian has undertaken a quest in fulfilment of his coronation oath to find the seven lost Lords of Narnia: Argoz, Bern, Mavramorn, Octesian, Restimar, Revilian, and Rhoop. Lucy and Edmund are delighted to be back in the Narnian world, but Eustace is less enthusiastic, as he has never been there before and had taunted his cousins with his belief that this alternate universe had never existed. The Talking Mouse Reepicheep is also on board, as he hopes to find Aslan’s Country beyond the seas of the “utter East”. When Eustace teases Reepicheep, much is revealed about the mouse’s pugnacious character.
They first make landfall in the Lone Islands, nominally Narnian territory but fallen away from Narnian ways: in particular the slave trade flourishes here, despite Narnian law stating that it is forbidden. Caspian, Lucy, Edmund, Eustace and Reepicheep are captured as merchandise by a slave trader, and a man “buys” Caspian before they even reach the slave market. He turns out to be the first lost lord, Lord Bern, who moved to the islands and married a woman there after being banished from Narnia by Miraz. When Caspian reveals his identity, Bern acknowledges him as King. Caspian reclaims the islands for Narnia, and replaces Gumpas, the greedy governor, with Lord Bern, whom he names Duke of the Lone Islands.
At the second island they visit, Eustace leaves the group to avoid participating in the work needed to render the ship seaworthy after a storm has damaged it, and hides in a dead dragon‘s cave to escape a sudden downpour. The dragon’s treasure arouses his greed: he fills his pockets with gold and jewels and puts on a large golden bracelet; but as he sleeps, he is transformed into a dragon. As a dragon, he becomes aware of how bad his previous behaviour was. He attempts to shed his dragon skin without success. It is only with the help of Aslan that he is able to become human again, though the process is very painful. Caspian recognises the bracelet: it belonged to Lord Octesian, another of the lost lords. They speculate that the dragon killed Octesian — or even that the dragon was Octesian. Aslan turns Eustace back into a boy, and as a result of his experiences he is now a much nicer person.
They narrowly escape being sunk by a sea-serpent and stop at Deathwater Island, so named for a pool of water which turns everything immersed in it into gold, including one of the missing lords who turns out to have been Lord Restimar. Then they land on the Duffers‘ Island, where Lucy removes an invisibility spell from the Duffers (later Dufflepuds) at their request and befriends the Magician who cast it. At the Island Where Dreams Come True – called the Dark Island since it is permanently hidden in darkness – they rescue a desperate Lord Rhoop. Eventually they reach the Island of the Star, where they find the three remaining lost lords in enchanted sleep. Ramandu, the fallen star who lives on the island, tells them that the only way to awaken them is to sail to the edge of the world and there to leave one member of the crew behind.
The Dawn Treader continues sailing into an area where merpeople dwell and the water turns sweet rather than salty, as Reepicheep discovers when he belligerently jumps in to fight a mer-man who he thinks challenged him. At last the water becomes so shallow that the ship can go no farther. Caspian orders a boat lowered and announces that he will go to the world’s end with Reepicheep. The crew object, saying that as King of Narnia he has no right to abandon them. Caspian goes to his cabin in a temper, but returns to say that Aslan appeared in his cabin and told him that only Lucy, Edmund, Eustace, and Reepicheep will go on.
These four named venture in a small boat through a sea of lilies until they reach a wall of water that extends into the sky. Fulfilling Ramandu’s condition, Reepicheep paddles his coracle up the waterfall and is never again seen in Narnia. Edmund, Eustace, and Lucy find a Lamb, who transforms into Aslan and tells them that Edmund and Lucy will not return to Narnia and that they should learn to know him by ‘another name’ in their own world. He then sends the children home.
In their own world, everyone remarks on how Eustace has changed and “you’d never know him for the same boy” – although his mother believes that Edmund and Lucy have been a bad influence on him in the way that they have made him “commonplace and tiresome”.