Percy describes another strange but mostly routine day at Meriwether College Prep, where he and his only friend, Tyson, are targets for bullying. During an English exam in the schoolyard, Matt Sloan tries to give Tyson a wedgie, and Tyson accidentally throws him into a tire swing. Sloan retaliates by insulting Tyson, which leaves Tyson crying and Percy angry. Later, in science, Tyson and Percy accidentally ace an explosion assignment, but Percy remains distracted by a bad dream and a growing fear that something is wrong at camp.

In social studies, Percy looks at a photograph of Annabeth because it reminds him that Camp Half-Blood is real and that she might understand his dream. Matt Sloan steals the picture, mocks Percy, and has his new friends destroy it. Percy notices the newcomers have bizarre names on admissions tags, and Tyson had already warned that they “smell funny,” but Percy does not understand the danger until PE begins.

At dodgeball, Sloan rigs the teams against Percy, putting most strong players and the mysterious visitors on his side. As soon as the game starts, the visitors throw with impossible force, then reveal themselves as giant Laistrygonian cannibals. They seal the gym doors, produce flaming bronze dodgeballs, and turn the game into a hunt. Coach Nunley notices nothing unusual because the Mist hides the monsters’ true nature from mortals, and Percy realizes he is defenseless because Riptide is in his jeans inside the locked locker room.

As the giants attack, Tyson repeatedly saves Percy and the other students by catching or batting back the burning balls. The returned shots destroy several of the monsters, proving Tyson has extraordinary strength and reflexes. Percy then uses the attack to his advantage by luring a fireball into the locker room door, which explodes and blows apart part of the wall. Tyson knocks out another giant, but Joe Bob, the last monster standing, blasts Tyson into the wall and prepares to finish him.

Percy tries to rush the giant and recover Riptide from his ruined jeans, but before Joe Bob can kill him, Annabeth suddenly appears and stabs the monster from behind. She punches Matt Sloan when he recognizes her from Percy’s photo, then hurriedly tells Percy she has been trying to reach him all morning and orders him to meet her outside with Tyson. She vanishes under her magic cap just as the headmaster, teachers, and police enter the burning gym. Sloan immediately blames Percy for the destruction, Coach Nunley vaguely backs him up, and Percy knows the adults will never believe the truth, so he grabs Riptide, tells Tyson to come with him, and escapes through the hole in the wall.