Percy sets out in search of his companion, the satyr Grover, who has fallen into the trap of the Cyclops Polyphemus. At stake is not only the satyr, but the entire “Camp Half-Blood”: someone has poisoned Thalia’s sacred tree, and that means the camp’s protection is about to collapse and nothing will be able to hold the monsters back from the camp.
Only the magical Golden Fleece, hidden on Polyphemus’s island, can heal the tree — and to reach it, one must sail across the Sea of Monsters and battle its inhabitants — Scylla and Charybdis, sweet-voiced sirens, and the cruel sorceress Circe.