The Sea of Monsters

Percy Jackson is trying to survive another miserable stretch at Meriwether Prep, where he and his only friend, Tyson, are constant targets for bullies. During a dodgeball game, the new students on Matt Sloan's side reveal themselves as Laistrygonian giants and turn the match into an attack with flaming bronze balls. Tyson repeatedly saves Percy with impossible strength, and Annabeth Chase suddenly appears to kill the last giant and drag Percy away. With the school in ruins and mortals blaming him, Percy flees with Annabeth and Tyson. Annabeth explains that Camp Half-Blood is in immediate danger, and the three take the Gray Sisters' supernatural taxi back to camp. During the wild ride, Percy forces the sisters to give him a cryptic clue: 30, 31, 75, 12.

At Camp Half-Blood, Percy finds the borders failing. Two fire-breathing Colchis bulls are already attacking on Half-Blood Hill, moving where monsters should not be able to go. Percy joins Clarisse La Rue and the other campers in the fight, but Tyson is the one who saves him by crossing into camp, surviving fire, and crushing a bull with his bare hands. Annabeth tells Percy to see past the Mist, and he realizes Tyson is a young Cyclops. After the battle, Percy learns the deeper cause of the crisis: Thalia's pine tree has been poisoned, and because its magic powers the camp's borders, the whole camp is slowly becoming vulnerable. Chiron has been blamed and exiled, Argus is gone, and the cruel Tantalus has taken charge.

The camp feels harsher and more frightened than before. Chiron privately tells Percy and Annabeth that the poison came from the Underworld and warns Percy not to rush into a trap. At dinner, Tantalus humiliates Tyson, but then a glowing trident appears above Tyson's head, claiming him as a son of Poseidon. Percy is horrified to learn Tyson is his half-brother, and the rest of camp mocks him for it. Percy responds badly, ashamed of Tyson even after Tyson's loyalty and kindness. Annabeth's own distrust of Cyclopes deepens the tension between them. Meanwhile, Grover begins appearing in Percy's dreams from a cave, where he reveals that he has been trapped by the Cyclops Polyphemus in the Sea of Monsters after following a mysterious nature scent that seemed connected to Pan. During a camp chariot race, Stymphalian birds attack, and Percy and Annabeth save the camp by using Chiron's music collection to drive the birds into range of Apollo's archers.

While being punished with kitchen duty afterward, Percy and Annabeth realize Grover may have found the Golden Fleece, the one object powerful enough to heal Thalia's poisoned tree. The Gray Sisters' numbers turn out to be coordinates in the Atlantic, near the modern Sea of Monsters. Percy pushes for an official quest, but Tantalus spitefully awards it to Clarisse instead. Frustrated and desperate, Percy is secretly visited by Hermes, who encourages him to go anyway. Hermes gives him magical supplies, including a wind-filled thermos, powerful vitamins, and travel bags. Percy leaves camp with Annabeth and Tyson, and they board the Princess Andromeda, where they discover Luke Castellan has built something far larger than a private rebellion. The ship is full of monsters, entranced mortals, and recruited half-bloods.

Percy, Annabeth, and Tyson confront Luke and learn the extent of his betrayal. Luke admits that he poisoned Thalia's tree and reveals a golden sarcophagus in which a titan lord is slowly reforming. He tries to persuade Percy and Annabeth that the gods deserve to fall, but they refuse. Luke orders them to be fed to a drakon, and the trio barely escapes in a lifeboat by using Hermes's wind thermos. On the way to safety, Percy and Annabeth begin to suspect that Luke may want them to retrieve the Fleece for his own ends. In Virginia, they briefly reach an old safe house Annabeth once shared with Luke and Thalia, which exposes how painful and complicated her history with Luke is. A Hydra attacks them near a Monster Donut shop, but Clarisse rescues them aboard her undead-crewed ironclad, the CSS Birmingham.

Clarisse refuses to cooperate because she wants to win the quest on her own, though Percy later overhears Ares pressuring her not to let Percy take it from her. As the ship tries to enter the Sea of Monsters by passing Charybdis and Scylla, Grover's dreams show that Polyphemus has the Fleece hanging on a tree and plans to force the wedding by the next day. Clarisse attacks Charybdis head-on, but the plan fails. Tyson goes below to save the overheating engine, the ship is thrown toward Scylla, and the Birmingham is destroyed. Percy is blasted into the sea believing Tyson has died.

Percy and Annabeth survive in a rowboat and speak more honestly than before. Annabeth finally tells Percy about the Great Prophecy: the next child of the Big Three to reach sixteen will decide whether Olympus is preserved or destroyed. Percy realizes Kronos may want to use him rather than simply kill him. Soon afterward, they land at Circe's island spa, where Percy is turned into a guinea pig. Annabeth resists Circe's attempt to recruit her, uses Hermes's vitamins to break the magic, restores Percy, and unleashes Circe's imprisoned victims, including Blackbeard. Percy and Annabeth escape aboard the Queen Anne's Revenge, which Percy can instinctively command at sea. On the voyage, Annabeth reveals that a Cyclops's deception once delayed her, Luke, Thalia, and Grover, helping lead to Thalia's death, which explains some of her fear and bitterness. At the island of the Sirens, Percy saves Annabeth from their song and learns that her fatal flaw is hubris: the belief that she could rebuild the world better than gods or heroes around her.

When they reach Polyphemus's island, Percy and Annabeth find a false paradise guarded by savage sheep and the Cyclops himself. They discover that Grover is alive, still pretending to be Polyphemus's bride, and that Clarisse is also captive. Annabeth distracts Polyphemus by calling herself Nobody while Percy sneaks into the cave under a sheep and frees Grover and Clarisse. During the escape, Polyphemus badly injures Annabeth. Percy, Grover, and Clarisse fight him, but Percy hesitates to kill him because Polyphemus's fear reminds him of Tyson. That mercy backfires when Polyphemus immediately betrays him. Then Tyson reappears alive, having been rescued by Rainbow the hippocampus. Tyson saves Percy, retrieves the Golden Fleece because the sheep accept him, and Percy uses it to revive Annabeth. As they flee, Polyphemus survives again and sinks their ship, but Percy and Tyson communicate through the sea and summon hippocampi to rescue everyone.

In Miami, Percy realizes Clarisse's prophecy means she must return to camp alone by air with the Fleece. He gives her the Fleece and their remaining money, trusting her to finish the quest. Luke immediately captures Percy, Annabeth, Grover, and Tyson on the docks and demands the Fleece, but Percy turns the situation around by provoking Luke into confessing through an Iris-message visible at camp. Luke admits poisoning Thalia's tree and proving Chiron innocent, so Dionysus dismisses Tantalus and restores Chiron. Luke then decides Percy must die, but Percy stalls him in a duel until Chiron and the wild Party Ponies raid the Princess Andromeda and rescue the group. Later, Chiron warns Percy that Kronos spared him because he hoped to recruit him, and reveals why others have long distrusted him: Kronos is Chiron's father.

Back at Camp Half-Blood, Clarisse hangs the Golden Fleece on Thalia's tree, and the poison is driven out. The borders recover, Clarisse is honored, and camp life begins to settle. Hermes visits Percy and explains that the gods often help indirectly; he also delivers a note from Poseidon containing only two words: Brace Yourself. Tyson gives Percy a shield disguised as a watch and explains that he once prayed to Poseidon for a friend, and Percy was the answer. During the resumed chariot race, Tyson's inventions help Percy and Annabeth win, and Percy finally, publicly calls Tyson his brother. Soon after, Tyson leaves to serve Poseidon in the Cyclopes' forges. The apparent victory does not last. The Fleece heals Thalia's tree too completely, removing the unnatural condition that bound her to it. Before dawn, Percy, Grover, Annabeth, and Chiron gather at the tree and find that Thalia herself has returned to life. Her restoration changes the meaning of the prophecy and gives Kronos a new path into the future.