Seven years before the events of the film, young
Annabeth Chase,
Thalia Grace,
Luke Castellan and
Grover Underwood were pursued by monsters on their way to
Camp Half Blood. When Thalia sacrificed herself to save her friends, her father,
Zeus, honored her act by turning her into a tree, which formed a protective barrier around the camp, keeping everyone inside within a safe zone. In the present day,
Percy Jackson now attends camp and has befriended Annabeth and Grover. His quest for Zeus's lightning bolt has faded in popularity and he is called a one-quest wonder by camp bully
Clarisse La Rue. He is joined at camp by a young
cyclops named Tyson, a previously unknown son of a sea-nymph and
Poseidon, and therefore Percy's half-brother. After camp is attacked by a
Colchis bull, the campers discover Thalia's tree has been poisoned by Luke and the barrier is weakening. Annabeth learns about the healing powers of the
Golden Fleece, which Grover senses is located off the coast of
Florida, and they propose a quest to retrieve it to use to heal Thalia's tree. As it will involve crossing the Sea of Monsters (known to humans as the
Bermuda Triangle), camp counsellor
Mr. D denies them, but then claims the idea as his own and selects Clarisse for the quest. Meanwhile, Percy consults the
Oracle of Delphi, who tells of a prophecy that a half-blood of one of the three elder gods will one day either save or destroy
Olympus.
Chiron informs Percy that as he is the only living human half-blood of the eldest gods, the Prophecy may refer to him. Percy, Annabeth, Grover, and Tyson decide to leave camp in pursuit of the Fleece. They ride in the Chariot of Damnation (a
New York City cab) as far as
Washington D.C., where its drivers — the
Graeae — eject them for lacking enough
drachmas. Demi-gods aligned with Luke kidnap Grover, and with the help of
Hermes, the others track him down to Luke's yacht, the "Andromeda". Luke imprisons the group and reveals his intention to use the Fleece to resurrect the
Titan Kronos and have him destroy Olympus. Percy uses his ability to manipulate water to free the group, and they escape aboard one of the yacht's lifeboats. The group crosses the Sea of Monsters and are swallowed by
Charybdis. Working together with Clarisse, who was also swallowed along with her ship, they escape by blasting their way out of Charybdis's gut, and Percy guides the ship towards the coast of Florida. Landing there, they find the Fleece guarded by the cyclops Polyphemus in his lair, as well as Grover, disguised as a female cyclops. They rescue Grover, retrieve the Fleece, and trap Polyphemus, but they are ambushed by Luke. He makes Percy hand over the Fleece, while Tyson is shot with a crossbow while trying to protect Percy and falls into a roaring stream. As Luke begins reviving Kronos, Percy frees everyone and the group engages Luke and his allies in combat. Percy loses to Luke but is saved by Tyson, who survived after being healed by the water. However, they are unable to prevent Kronos's resurrection, and Percy engages him in battle, eventually using his sword to defeat and re-imprison the titan. Annabeth is impaled by the
Manticore and dies, but is resurrected by the Fleece. The group returns to camp, where Chiron expresses uncertainty as to whether Percy has fulfilled the prophecy by preventing Luke's plan to destroy Olympus. Clarisse places the Fleece on Thalia's tree, which restores the barrier, but also resurrects Thalia. Percy realizes that, as Thalia is the daughter of Zeus, the prophecy of Olympus could now also refer to her. ==Cast==