"The Battle of the Labyrinth" is the fourth book in the series. Annabeth and Percy find entrance into the Labyrinth during a game of capture the flag. Percy soon learns that Luke had used the entrance. He will lead the Titan army through the Labyrinth straight into the camp. To get into the Labyrinth, Percy has to find the Delta Symbol (Δ) and touch it. Using the Labyrinth, Percy tries to find Daedalus so Luke cannot get Ariadne's string. This would prevent Luke from attacking the camp.