Thalia's tree, the magical border of Camp Half-Blood, has been poisoned. Chiron is blamed for the poisoning because he is the son of Kronos, and is dismissed from camp. The only thing able to rid the tree of its poison is the Golden Fleece, which is located on the island of Polyphemus, in the Sea of Monsters, also known by mortals as the Bermuda Triangle.
Clarisse La Rue, the daughter of Ares and enemy of Percy since his first day at Camp Half-Blood, is given the quest to go to the Sea of Monsters to find the Golden Fleece. Percy and Annabeth also decide to go, not just for the Golden Fleece, but for their friend Grover Underwood, who is trapped in Polyphemus' cave. Tyson, a young Cyclops whom Percy befriended, joins them on their journey after saving Camp Half-Blood from two metal, fire-breathing bulls. He is also Poseidon's son and Percy's half-brother.
As they enter the Sea of Monsters, they have to get past Scylla and Charybdis. Instead of trying to sail in between the two, Clarisse heads towards Charybdis. After that, Percy and Annabeth journey to many dangerous islands, and Annabeth tells Percy many things about how Thalia died and she also mentions her prophecy. She also tells Percy that he has a choice to make when he becomes 16 years old, how a child of one of the "Big Three" has to make a choice whether or not to save the world. That was the reason the "Big Three" swore off children. Fighting their way through many other obstacles, such as the Sirens and Circe's island, they join up with Clarisse and Grover, who had been captured by Polyphemus, and later Tyson. After fighting Polyphemus, they leave with the Golden Fleece.
When they return to the mortal world, they send off Clarisse on an airplane with the Fleece to camp alone. Percy, Annabeth, Grover and Tyson are kidnapped by Luke. In a duel with him, Percy is nearly killed. He is saved by Chiron, who is proven innocent, and his relatives, the Party Ponies. When they return to camp the Fleece is put on the tree. The Fleece not only revives tree, but Thalia too, who appears as she looked in Percy's dreams. It turns out that this was actually Kronos's plan, done so that he would have another chance to manipulate the prophecy which governs the future of Olympus and the Twelve Olympians. At the end, Percy and Annabeth win a chariot race and Annabeth kisses Percy on the cheek. Then in a dream, Tyson is sent a message from Poseidon that he can go to the underwater palace to learn more of forging and smithing.
