Unpleasant, know-it-all bully Eustace Scrubb is transported, along with his cousins Edmund and Lucy Pevensie, to the magical world of Narnia through a painting of a ship. Swallowed up by the picture, the three children find themselves falling into the water beside the Dawn Treader, a sailing ship built by King Caspian of Narnia for the purpose of traveling east to find seven missing lords exiled by his evil Uncle Miraz. Caspian is also searching for the eastern edge of the world (Narnia is apparently flat) and has brought with him Reepicheep the Talking Mouse, who, according to a prophecy, will find what he seeks in the "utter East."
Edmund and Lucy, who are also Narnian royalty, greet their old friend King Caspian. All three children become members of the ship's company, although Eustace does so with bad grace. Eustace clashes with Reepicheep, attempting to use his greater size to tease and torment the mouse. However, Reepicheep's ferocity and honor eventually make it clear to Eustace that he'll have to stop if he doesn't want to find himself in a swordfight.
The Dawn Treader arrives at the three Lone Islands, Felimath, Doorn, and Avra. When they go ashore on their own for a walk, Eustace, Lucy, Edmund, Caspian, and Reepicheep are captured by slave traders. Caspian is sold to a local nobleman who turns out to be Lord Bern, one of the seven missing lords. With Lord Bern's help, Caspian overthrows the corrupt Governor Gumpas, abolishes the slave trade in the Lone Islands, and rescues his friends. The Dawn Treader's crew restocks and refurbishes the ship. Caspian makes Bern a Duke and installs him as the new ruler of the Lone Islands, under Narnian dominion. (Found Lord Count: One!)
When the Dawn Treader sets sail again, it's caught up in a violent storm that destroys the mast and practically totals the ship. After several days on low water rations, they land at what appears to be a deserted island to make repairs. While the others are working, Eustace sneaks away to rest and gets lost among foggy hills. Finding himself in a strange valley, he witnesses a dragon die from old age. After making sure the dragon is dead, Eustace ventures into its cave, where his greed is awakened by the sight of a massive amount of treasure. Eustace puts on a gold bracelet and falls asleep lying on a pile of gold coins. When he wakes up he is shocked to discover that he himself has turned into a dragon, with the gold bracelet digging painfully into his now-enormous and scaly arm. Eustace returns to his shipmates and manages to communicate what has happened to him. As a dragon, he is able to help hunt for food and uproot trees for rebuilding the ship.
When the Dawn Treader is nearly ready to leave, Eustace has a mysterious, miraculous encounter with the great lion, Aslan, who turns him back into a human boy by peeling layers of scaly skin off of him and tossing him into a pool of healing water. Eustace rejoins his shipmates a much more pleasant and helpful person. Examining the bracelet that Eustace was wearing, Caspian realizes that it belonged to one of the seven missing lords, Octesian, who must have been killed by the previous dragon or transformed into one the way Eustace was. (Found Lord Count: Two!)
After setting sail again, the Dawn Treader is attacked by a sea serpent that tries to encircle the ship in a coil of its snakelike body and squeeze it to pieces. Thanks to the bravery of Reepicheep and Eustace and the efforts of everyone on board, the sea serpent is pushed off the end of the ship, and the tightening loop of its body crashes harmlessly into the sea.
The next island the Dawn Treader lands at appears to be uninhabited. Caspian, Edmund, Eustace, Lucy, and Reepicheep go for a hike and discover a pool of water in a valley. When they sit down to rest beside the pool, they discover the belongings of one of the missing Narnian lords, including his sword and chain mail shirt (a "chain mail shirt" is one of those metal shirts made out of chains that medieval knights and Frodo Baggins wear). They notice a golden statue of a man at the bottom of the pool, and when Edmund tries to measure the depth of the water with his spear, it turns to gold, too. They realize that the water of this pool and spring turns everything to gold. At first Caspian and Edmund become greedy and cruel as they imagine the power this water could bring them, but when Aslan appears briefly on a nearby hillside, they come to their senses. They return to the ship with only a fuzzy memory of their experience, although they remember that they found one of the missing lords (they don't yet know which one). (Found Lord Count: Three!)
Next the Dawn Treader lands on an island where no people are visible, but there are clear signs of civilization – short mowed lawns, a water pump, and a large two-story house. Lucy overhears some invisible people plotting to attack the Narnians on their way back to the shore. Strangely, instead of hearing footsteps when they move, Lucy hears large thumps and sees clouds of dust rise. She warns Caspian and the others, and they go down to the beach prepared for a fight. They are met by the invisible Duffers, a tribe of foolish people who agree with everything their equally foolish Chief says. The Chief explains that the island is ruled by a magician who made them ugly; in retaliation, they made themselves invisible. However, the magician has disappeared and they have grown tired of invisibility. Only a young girl can go upstairs in the magician's house and reverse the spell, and they insist that Lucy must do this or they will slaughter the Narnians. Lucy agrees. The Duffers feed them dinner and everyone rests for the night.
In the morning, Lucy goes upstairs in the magician's house, overcoming her fear of its creepy occult symbols and long-abandoned corridor. Reading the magician's book, she is tempted to use a spell to make herself beautiful. She resists when she sees Aslan's face appear on the page, but she succumbs to the temptation to use another spell to spy on her friends back home. Finally she finds and casts the anti-invisibility spell. Aslan becomes visible beside her and talks to her about eavesdropping. Then he introduces her to the magician Coriarkin, who has also been made visible.
Coriarkin feeds Lucy a meal of her favorite foods from back home in England and they talk about the Duffers. Lucy begins to realize that the magician is a kind and thoughtful ruler, and the Duffers are extremely foolish. Looking out the window, she sees that the now-visible Duffers are Monopods – one-footed dwarfs. She tries to explain to them that she doesn't find them ugly, but they won't listen.
Coriarkin tells Caspian that a ship with four lords on it stopped at his island several years ago. By comparing the lords Coriarkin met to the ones they have found so far, Caspian figures out that the man who was turned to gold was Lord Restimar. The Narnians and Dufflepuds, as the people come to call themselves, say goodbye and the Dawn Treader sails on.
Next the Dawn Treader encounters a strange cloud of darkness hovering over the water. Nobody really wants to sail into it, but Reepicheep accuses the men of being chicken, so they decide to take a look inside. The darkness is silent and creepy; the only things visible are lanterns hanging on the ship. The adventurers hear a man screaming in the distance and they bring him on board. He tells them to flee and explains that they are sailing toward the island where dreams come true – not daydreams or hopes, but actual dreams, including their worst nightmares.
That sounds pretty bad, and the crew can't navigate and seem to be going around in circles. Each person on the ship starts to hear noises they recognize from their worst dreams. Lucy calls on Aslan and a white albatross appears and leads the ship out of the black cloud. The man they have rescued introduces himself as Lord Rhoop, another of the exiled Narnian lords. He begs Caspian never to make him return to the Dark Island, but he has nothing to worry about – when they turn around, the darkness has disappeared as though it never existed. (Found Lord Count: Four!)
The Dawn Treader lands at a beautiful, hilly island. When they go ashore, Caspian and his friends discover a clearing surrounded by pillars, in the center of which is a long stone table covered in a delicious feast. At the table are the last three of the missing lords. They have fallen into an enchanted sleep, and their hair and beards have grown to cover the table in a tangled mass. Caspian, Lucy, Edmund, Eustace, and Reepicheep sit at the table all night, waiting to see what will happen.
Near dawn a beautiful blonde girl in a blue dress comes out of a door in the hillside. She greets them and welcomes them to Aslan's Table, asking why they aren't eating. They explain that they were concerned that the food had put the lords into an enchanted sleep, but she tells them the lords have never tasted it. Caspian asks how to break the spell, and the girl says her father will teach him.
At that moment her father, an old man with silver hair who seems to emit light, emerges from the hillside. His name is Ramandu. Ramandu and his daughter face the east and sing as the sun rises. A flock of birds flies out of the sun and consumes all the leftover food. Ramandu and his daughter explain that the lords argued about whether to sail further east, sail back home, or stay on the island, and when one of them touched the Stone Knife lying on the table they were enchanted. (It's the same stone knife that was used to kill Aslan in The Lion, the Witch, and the Wardrobe.) To break the spell, they must sail as far east as possible and leave at least one person behind. Caspian agrees to do this, and with the help of Drinian and Rhince, he convinces all but one of the crew to come with him. (Found Lord Count: Seven!)
The Dawn Treader sets out to sail to the extreme eastern edge of the world. At one point, Lucy sees warlike Sea People in the water, but Drinian warns her not to tell the men about them so that the men aren't tempted to jump overboard. Reepicheep, feeling challenged by the warlike King of the Sea People, does leap into the ocean, but is quickly distracted by discovering that the water is fresh and sweet, not salty. This is a sign that the prophecy spoken over him when he was a baby is coming true. Everyone drinks the sweet water and feels extremely healthy.
As they get farther east, they get closer to the sun (remember, Narnia is flat) and everything gets almost unbearably bright, but drinking the water seems to help them withstand the light. The ship sails into a mass of white water lilies growing in all directions. Soon, they come to a place too shallow for the ship to continue. Caspian wants to go on with Reepicheep, but Aslan reminds him that it is his duty go to back and rule Narnia. Reepicheep, Edmund, Eustace, and Lucy set out in the boat, while the Dawn Treader sails back west toward Ramandu's island.
For three days, the four remaining adventurers journey east. Eventually their boat runs aground in a shallow area, and they find themselves looking at a wall of water 30 feet high flowing upwards. Behind it they believe they can see the enormous green mountains of Aslan's country. Reepicheep goes on toward the wall of water in his small coracle (a lightweight boat). He is borne upwards and disappears. For reasons they can't explain, the three children get out of the boat and wade south. Eventually they come to a rolling green plain where the sky comes down to meet the grass as a glassy blue wall. They meet a lamb who offers them a meal of roasted fish and then reveals himself to be Aslan. Aslan sends the children back to their own world, explaining that they must get to know him there under another name. He tells Edmund and Lucy that they will never return to Narnia but leaves open the possibility that Eustace might come back some day.
The narrator leaves us with two points: Caspian marries Ramandu's daughter and she becomes a great Narnian queen, and Eustace has permanently changed for the better.