Perseus "Percy" Jackson is a twelve-year-old New Yorker who has dyslexia and ADHD. For reasons unknown, he often finds himself in unique situations that get him kicked out of school. During a school trip to a museum, a bully, Nancy Bobofit, bullies Percy’s friend Grover. At the same time Percy defends his friend, the fountain that they were next to grabs Nancy. However, Nancy believed that she was pushed into the fountain, and lies about the incident to Mrs. Dodds, Percy's pre-algebra teacher. Mrs. Dodds separates Percy from the rest of the class before she transforms into one of the Furies from Greek mythology and attacks him. Mr. Brunner, Percy's Latin teacher, tosses a pen to Percy, and when uncapped, it transforms into a sword. Percy kills Mrs. Dodds, and when he returns to the class, no one seems to remember the pre-algebra teacher. Only Grover seems suspicious when Percy confronts him.
Percy seeks Mr. Brunner for extra help on Latin for the final the following week. However, he unwittingly hears Grover and Mr. Brunner talking about him, and about a stolen item that was important.
Percy goes home on the last day of school, and his mother Sally takes him to the beach house where Percy's parents met long ago. Unfortunately, that night, a storm rages, and Grover warns them to flee. They are attacked by the Minotaur while driving to the countryside, and in the ensuing fight, Sally disappears in a gold flash when the monster grabs her. Enraged, Percy kills the Minotaur and passes out soon afterwards.
Three days later, he wakes up in the infirmary of Camp Half-Blood, where Grover and Mr. Brunner - who is Chiron, the camp director – tell him that he is a demigod - half-mortal, half-god - but his godly parent is unknown. For this reason, he is sent to the Hermes cabin, which receives all the unclaimed demigods. During his time there, he befriends Luke, the cabin leader and son of Hermes, becomes an acquaintance of Annabeth, a daughter of Athena, and an enemy of Clarisse, who is a daughter of Ares. Through a series of events during Percy’s stay at camp, Poseidon claims him.
However, just as the trident that claimed him fades away, a hellhound bursts into camp and sends the campers into panic, as these monsters supposedly are unable to enter. To protect the camp, Chiron sends Percy on a quest.
Percy is eventually tasked by the Oracle to retrieve Zeus' stolen lightning bolt, mistakenly believed to have been taken by Poseidon. Poseidon, however, blames Hades for stealing it, in hopes of starting a war. Annabeth and Grover go west with him in order to retrieve the stolen items. However, upon reaching Hades’s lair, Hades accuses Percy of stealing his helm, threatening to kill him and his mother before releasing the dead into the world. Percy and his friends escape, and they find out that Ares had the stolen items all along. Percy fights the god of war in a duel and surprisingly wins. Percy returns the helm to Hades, and in return, Hades returns Sally safely.
After Percy returns the lightning bolt to Zeus, Luke reveals that he was the thief and is working for Kronos, the Lord of the Titans. He tries to kill Percy by calling on a pit scorpion, but Percy survives. At the end of summer, Percy ultimately chooses to return home for the school year before he comes back for the next summer.