A Normal School Year, Until the Monsters Arrive
Percy Jackson has a normal seventh-grade year at Meriwether College Prep, despite recurring dreams of Grover in trouble. His only real friend is Tyson, a large, homeless boy who seems impervious to insults. During a dodgeball game against new students, Percy realizes they are Laistrygonian giants when they begin throwing flaming projectiles. With Tyson's incredible strength and Annabeth Chase's timely arrival, Percy defeats the giants. Annabeth tells Percy that Camp Half-Blood is under attack and in danger, so they must flee. During their escape, Tyson reveals himself to be a Cyclops, much to Percy's shock and Annabeth's discomfort.
Camp Half-Blood Under Siege and a Prophecy Revealed
Percy, Annabeth, and Tyson arrive at Camp Half-Blood to find its magical borders failing, allowing monsters to breach the defenses. Thalia's pine tree, which provides the camp's protective barrier, has been poisoned. Chiron has been fired, replaced by Tantalus, a spirit from the Underworld who is cursed with eternal hunger and thirst and holds a grudge against demigods. Clarisse La Rue, daughter of Ares, gets a quest to find the Golden Fleece, believed to be the only cure for Thalia's tree. Percy believes he should go, haunted by a prophecy from the Oracle that mentions a child of the Big Three saving or destroying Olympus.
A Dangerous Journey to the Sea of Monsters
Despite Tantalus's refusal to grant him a quest, Percy decides to go after the Fleece with Annabeth and Tyson. Hermes helps them, providing supplies like thermoses containing the winds and a magical multivitamin. They hail a magical taxi, the Chariot of Damnation, driven by the three Graeae, who share one eye. Percy gives them a drachma and briefly takes their eye for information about their destination – the Sea of Monsters. He learns the coordinates: 30, 31, 75, 12. They escape the taxi, which is a monster, and find themselves at a cruise ship called the Princess Andromeda, which Percy recognizes from his dreams.
Encounter with Luke and the Princess Andromeda
On board the Princess Andromeda, Percy, Annabeth, and Tyson discover it is Luke Castellan's base. Luke reveals his plan to resurrect Kronos, the Titan Lord, using the Golden Fleece. He captures them and tries to turn Percy against the gods, explaining his resentment towards them. Luke shows a golden sarcophagus where Kronos's essence is slowly re-forming. With Tyson's strength and Annabeth's quick thinking, they escape Luke's forces, including a Manticore, by jumping overboard. A group of Hippocampi, led by a female named Rainbow, rescue them from the ocean. They recognize Tyson and take them towards their destination.
Circe's Island and the Sirens
The Hippocampi take Percy, Annabeth, and Tyson to a spa resort island, which is Circe's island. Circe, a sorceress, tries to transform Percy into a guinea pig, successfully doing so before Annabeth, using Hermes' multivitamin, transforms him back. They escape Circe's island and continue their journey. Annabeth wants to visit the Sirens' island, hoping to hear their song and gain insight into her greatest desires and fears. Percy reluctantly ties her to the mast, but she breaks free and is tormented by visions before Percy rescues her. He learns about her insecurity and desire for her parents' approval and a better world.
The Clashing Rocks and Clarisse's Arrival
Their journey leads them to the strait between Charybdis and Scylla, the Clashing Rocks, a dangerous passage. They decide to navigate through Charybdis, hoping it will be less destructive than Scylla. During the attempt, Tyson falls overboard and is presumed dead, devastating Percy. Soon after, they encounter Clarisse's ship, the CSS Birmingham, powered by Confederate zombie sailors. Clarisse, having failed to navigate the strait, is in trouble. Percy and Annabeth board her ship, and Percy reveals himself, convincing Clarisse to work together. He helps fix her ship, and they resume the quest, mourning Tyson's loss.
Polythemus's Island and Grover's Predicament
Following the coordinates from the Graeae, they arrive at Polyphemus's island, where the Golden Fleece is. They find Grover, captured by Polyphemus and mistaken for a female Cyclops, and also discover the Golden Fleece. Polyphemus is a giant, one-eyed Cyclops who guards the Fleece. Percy, Annabeth, and Clarisse plan to rescue Grover and retrieve the Fleece. They use their wits and strength, with Annabeth distracting Polyphemus while Percy and Clarisse try to free Grover and get the Fleece. During the confrontation, Tyson reappears, rescued by a Hippocampus, much to Percy's relief and joy.
Retrieving the Fleece and a Difficult Escape
Tyson's unexpected return helps. He fights Polyphemus with Percy, using his Cyclops strength to overpower the giant. Annabeth's quick thinking and Grover's knowledge of the island also help them succeed. They blind Polyphemus with a stick, like Odysseus's trick, and retrieve the Golden Fleece. Escaping the island with the Fleece and the now-freed Grover is another challenge. They face more of Polyphemus's sheep and navigate the terrain, making their way back to Clarisse's ship, the Birmingham, which waits for them near the shore.
A Race Against Time and Luke's Ambush
With the Golden Fleece secured, Clarisse is tasked with returning it to Camp Half-Blood by Ares, who appears to her in a dream. Percy, Annabeth, Tyson, and Grover travel separately back to camp, unsure if they will make it in time. Luke intercepts them on the Princess Andromeda, demanding the Fleece. A battle happens on the ship. Percy and Luke duel, where Luke reveals his full power and loyalty to Kronos. Luke wants to stop Percy and get the Fleece for his own purposes, using it to empower Kronos.
The Fleece's Power and a Shocking Reawakening
Luke wounds Percy with a poisoned blade, but they escape, thanks to Tyson's intervention and Annabeth's quick thinking. Clarisse successfully delivers the Golden Fleece to Camp Half-Blood just in time. The Fleece's magic immediately begins to heal Thalia's poisoned tree, restoring the camp's protective barrier and driving away the monsters. The Fleece's healing power has an unforeseen side effect: Thalia Grace, daughter of Zeus, who was turned into a pine tree years ago to save her life, is resurrected as a human. This reawakening changes the Great Prophecy, as there are now two children of the Big Three (Percy and Thalia) who could fulfill it.