It ends with Percy having to leave his mother in the Underworld because he only has three pearls. So, he goes back up to the real world where he finds out the Ares was the one who had the Helm of Terror. Percy battles Ares hence the name of the chapter "I Battle my Jerk Relative" and he wins. The Furies get the Helm back from Percy with gratitude. The kids get to fly home where Percy tells Annabeth and Grover to go back to camp to tell them the news. He goes up to Olympus on the 600th floor and returns the bolt. After that, he goes back to his home in Manhattan where he finds his mother- Hades was feeling generous and Percy did return the Helm- and Gabe tells him to get out. He decides to go back to camp, but when he goes inside his room with his mom, he sees the package he sent to Olympus- Medusa's head. He tells his mother to use it and then leaves for camp. At the end of the summer he needs to decide if he is going to be a year-rounder or only a summer camper. He decides to be a summer camper because Annabeth was going to try out living with her father again. He goes home and Gabe is gone; his mother took his advice. That is how it ended.