In this book, Percy Jackson, who is the son of Poseidon, tries to prevent Luke Castellan, the son of Hermes, from attacking Camp Half-Blood.
Percy and his friends find that a new sword master, Quintus, has joined the camp. During a battle drill, Annabeth and Percy discover an entrance to the Labyrinth created by Daedalus. They soon learn that Luke has used this entrance before, and that he plans to lead his army through the Labyrinth to the camp through this entrance.