Percy's quiet seventh-grade year shatters when a dodgeball game turns into a monster ambush and a new friend, the awkward and powerful Tyson, turns out to be more than he seems. Worse news waits at Camp Half-Blood: the magical pine tree that shields the camp's borders has been poisoned, leaving its defenses failing and monsters slipping through. Only one thing can heal it, the legendary Golden Fleece, hidden somewhere in the treacherous waters of the Sea of Monsters. With Grover missing and a rival demigod already dispatched on the official quest, Percy, Annabeth, and Tyson set out on an unauthorized rescue of their own. Their voyage carries them through waters teeming with mythological perils, from a luxury-cruise nightmare to the enchantress Circe's island to the deadly song of the Sirens, all while Percy grapples with a more personal shock: Tyson is a Cyclops, and his half-brother. The journey forces Percy to confront his embarrassment over family, to reckon with prejudice and loyalty, and to learn that strength comes in unexpected forms. Lighter and faster than its predecessor while deepening the saga's central mysteries, the book sets ominous pieces in motion as the threat of the Titan lord Kronos grows clearer and a long-buried prophecy edges closer to its hour.