Desde que sabe que es hijo de un dios y una mortal, Percy Jackson espera que el destino le depare continuas aventuras. Y su expectativa se cumplirá con creces. Aunque el nuevo curso en la Escuela Meriwether transcurre con inusual normalidad, un simple partido de balón prisionero acaba en batalla campal contra una banda de feroces gigantes. A partir de ahí, las cosas se precipitan: el perímetro mágico que protege el Campamento Mestizo es destruido por un misterioso enemigo y la única seguridad con que contaban los semidioses desaparece. Así, para impedir este daño irreparable, Percy y sus amigos inician la travesía del temible Mar de los Monstruos en busca de lo único que puede salvar el campamento: el Vellocino de Oro. But things don’t stay quiet for long. Percy soon discovers there is trouble at Camp Half-Blood: the magical borders which protect Half-Blood Hill have been poisoned by a mysterious enemy, and the only safe haven for demigods is on the verge of being overrun by mythological monsters. To save the camp, Percy needs the help of his best friend, Grover, who has been taken prisoner by the Cyclops Polyphemus on an island somewhere in the Sea of Monsters, the dangerous waters Greek heroes have sailed for millennia—only today, the Sea of Monsters goes by a new name…the Bermuda Triangle. Now Percy and his friends—Grover, Annabeth, and Tyson—must retrieve the Golden Fleece from the Island of the Cyclopes by the end of the summer or Camp Half-Blood will be destroyed. But first, Percy will learn a stunning new secret about his family—one that makes him question whether being claimed as Poseidon’s son is an honor or simply a cruel joke.