The Camp Half-Blood is protected by Thalia, a daughter of Zeus that sacrificed her life to save the young Annabeth, Grover and Luke from an attack of Cyclops and was transformed by her father in a magical tree. Out of the blue, Percy Jackson is visited by his unknown half-brother Cyclops Tyson and they discover that Thalia was poisoned and is dying. Then the camp is attacked by a Colchis Bull but it is vanquished by Percy. Soon he learns that Luke is the one that has poisoned the magical tree. Annabeth discovers the mystic Golden Fleece is capable to heal Thalia and save their camp. But Mr. D assigns the winner demigod Clarisse to the quest of retrieving the magic fleece. However Percy Jackson, Annabeth, Grover and Tyson decide to follow an ancient prophecy and they go to the dangerous journey to the Sea of Monsters to recover the Golden Fleece from Luke that wants to revive the evil Kronos to destroy the Olympus.