Percy Jackson is a 13 year-old demigod. He is playing dodgeball against large seventh graders. He discovers they are Laistrygonians (a kind of monster in Greek mythology). They try to kill Percy with explosive fireballs. He is protected by Tyson, a friend. The Laistrygonians' fireballs do nothing to Tyson, which surprises Percy. Annabeth rescues Percy and takes Percy and Tyson to Camp Half-Blood. They find the camp is under attack. This is because Thalia's tree, which protects the camp, has been poisoned by an unknown intruder. Chiron, the activities director, has been sacked. This is because he is suspected to have poisoned the tree. He is replaced by Tantalus, a spirit from the Fields of Punishment.