The novel begins with Percy dreaming about Grover being hunted by a monster. He wakes on the last day of his 7th grade year, a peaceful year at Meriwether College Prep. There he has become friends with Tyson, a homeless child that the school has taken on as a form of charity. In gym class, during a game of dodgeball, Percy is attacked by a gang of Laestrygonians, but is saved by Tyson and Annabeth, who was on her way back to Camp Half-Blood after having dreams about the Camp in danger. The three take a magical taxi driven by the Graeae to Long Island. Upon reaching the camp, they see a group of campers lead by Clarisse LaRue battling a pair of Colchis bulls. Tyson is granted permission to move past the camp's boundary in order to save Percy once again. It is revealed that he is a baby Cyclops and a half brother of Percy, as he is also a son of Poseidon. An unknown enemy has weakened the protective walls of Camp Half-Blood by poisoning the tree of the demigod Thalia, leaving the campers vulnerable to future monster attacks. Camp counsellor Chiron is accused of poisoning the tree and fired, though the only evidence against him is that his father is Kronos. Before leaving, Chiron notes that only the Golden Fleece could save the camp.