Percy Jackson, the son of the sea god Poseidon, is adapting to life at Camp Half-Blood, a sanctuary for demigods like him. When the camp's magical borders are compromised by a mysterious poison, Percy and his friends discover that the only thing that can restore their protection is the mythical Golden Fleece. However, the fleece resides in the perilous Sea of Monsters, known to humans as the Bermuda Triangle. The camp's director, Dionysus, assigns the quest to another camper, but Percy feels a personal stake in the mission. Troubled by a prophecy that suggests his role in either saving or destroying Olympus, Percy is further shaken by the arrival of Tyson, a homeless boy who is revealed to be his Cyclops half-brother.

Defying camp orders, Percy sets out with his friends Annabeth and Grover, and his newfound brother Tyson, to find the Golden Fleece. Their journey is fraught with danger as they face a variety of mythical creatures, including a harrowing encounter with the taxi service of the three Gray Sisters, who provide them with vital information. As they delve deeper into their quest, they are challenged not only by the monsters and the treacherous sea itself but by rivalry with fellow demigods and an overarching threat from Luke Castellan, a former camper with sinister plans for Olympus. Luke's own quest for power involves the Fleece and reviving an ancient force that could bring destruction upon the world.