On Percy's last day of school, he and his friend/cyclops half-brother Tyson are attacked by giants. Annabeth appears and saves them before leaving for camp. When they arrive, Percy learns that Thalia's tree, the tree protecting the camp, has been poisoned and Chiron has been accused of poisoning and he was fired. If Thalia's tree fully dies, then the camp boarders won't be protected and more Monsters will start attacking the camp. This will continue unless they find another way to protect the camp borders or the camp will be overrun by monsters. To save the tree, a quest is given to find the Golden Fleece. Clarisse La Rue, a daughter of Ares, will lead the quest to find the Golden Fleece. Percy, Annabeth, and Tyson sneak off, not just for the Golden Fleece, but to find their friend Grover Underwood. They meet Luke on the ship, but are able to escape. They soon join with Clarisse on her quest. Her ship is destroyed passing through the Sea of Monsters and Annabeth and Percy continue their search for Grover after becoming separated from the others. After encountering the sirens and Circe, Annabeth and Percy reach the Polyphemus' cave. They are able to rescue Grover and Clarisse and Tyson helps defeats Polyphemus. They recover the fleece and head to Miami. Clarisse heads to camp and the others are captured by Luke. They are about to be killed before Chiron and the Party Ponies (other centaurs) rescue them and take them to camp. The Fleece is able to save the tree, but it also revives Thalia, whose spirit was in the tree.