Percy Jackson returns to Camp Half-Blood and, to his horror, discovers that Thalia's magical tree, which contains the soul of Zeus's daughter, has been poisoned, and it was its magic that has always protected the camp from monsters. Percy has visions from which he learns that to save the camp he must obtain the Golden Fleece. But getting to it is not so easy! It is kept where there is no way back - on the island of the Cyclops Polyphemus in the Sea of Monsters...