During their argument, his math teacher turns into a Fury, or goddess of vengeance, and Percy kills her with a pen that has turned into a sword. When they return to school, nobody remembers the teacher.
After school lets out for the summer, Percy and his mother are at the beach. A storm goes through town, and Percy's friend Grover encourages Percy and his mother to leave the house. During the storm, they are attacked by a Minotaur, or Greek monster, and Percy's mother, Sally, disappears. Percy kills the Minotaur and falls asleep.
When he wakes up, he is at Camp Half-Blood, a place for kids who have one human parent and one god parent. He doesn't know who his dad is at this point. Chiron, the head of camp, sends Percy on a quest to get Zeus' (a Greek god's) stolen lightning bolt that everyone thinks Poseidon, the god of the sea, stole. During this quest, Percy learns that Poseidon is his father. At the end of the book, Percy is able to bring the lightning bolt back to Zeus. He returns home to his normal school, though he plans to go back to Camp Half-Blood the following