Thalia's tree is poisoned at Camp Half-Blood, causing the camp's magical borders to fail. Percy and his friends must find something powerful enough to save the camp from invading monsters. They learn they must sail to the Sea of Monsters to find this item. During their quest, Percy fakes a rescue and learns a secret about his family. The book follows Percy, Grover, Annabeth, and Tyson as they embark on a quest to the Sea of Monsters to save Camp Half-Blood.