After Percy is kicked out of school, his mother takes him to Camp Half-Blood, a camp in New England where demigods are trained to use their powers. While at camp, Percy discovers that he is the son of Poseidon, god of the sea, and a nephew of Zeus and Hades. Percy also learns that Zeus has accused him of sealing his Master Bolt. Percy goes on a quest to retrieve the bolt from the most likely thief, Hades, ruler of the underworld, before the upcoming summer solstice.
Quest Across America
Percy, along with Annabeth, daughter of Athena, and Grover, a satyr, must travel across the country to the entrance to the Underworld in Los Angeles, California. They must travel by land, because flying may draw the attention of Zeus. Using buses, trains and taxis, the trio travel across the United States, stopping in St. Louis, Denver and Las Vegas. Along the way they battle Furies and Medusa, are attacked by Procrustes, the stretcher who stretched travelers to fit a bed, and meet Ares, god of war, who gives them supplies.
Journey to the Underworld
Percy and friends find the entrance to the underworld at DOA Records in Los Angeles. They bribe Charon, the ferryman, to take them across the River Styx. They find Hades' house and confront him about the theft of the Master Bolt. Hades, however, did not steal the bolt but has had his own Helm of Darkness stolen as well. During a fight with Hades, Percy discovers the bolt in the backpack that he received from Ares. Percy his friends flee the Underworld with the bolt to return it to Mount Olympus.
Fight with Ares
Ares confronts the group on the beach and confesses to stealing the bolt and helm from the original thief, in an attempt to spark a war between Hades, Zeus and Poseidon. Percy battles Ares and, wounding him, wins the Helm of Darkness. Hades’ Furies appear and collect the helm, acknowledging that Percy was not the thief.
Returning the Bolt
The trio decide to fly from California to New York City, the modern home of Mount Olympus to return the bolt in time. Percy send his two companions back to Camp Half-Blood and goes to Olympus alone and returns the bolt to Zeus. Percy also meets his father, Poseidon, who acknowledges him as his son.
The Real Thief
Percy returns to Camp Half-Blood as a hero. He spends the rest of the summer at the camp training with the other demigods. At the end of the summer, Luke, son of Hermes, confronts Percy and confesses to stealing the bolt and helm before attempting to kill Percy with a scorpion. Luke escapes from the camp while Percy recovers from the sting. Percy decides to return to New York for the school year, instead of staying at Camp Half-Blood throughout the year.