Percy Jackson, a twelve-year-old boy with dyslexia and ADHD, narrates The Lightning Thief. He lives in New York City, and when the story begins, he is on a field trip to the Metropolitan Museum of Art to see a Greco-Roman exhibit. A bully picks on his friend Grover Underwood. Percy pushes the bully into a fountain, and is taken away by his teacher, Mrs. Dodds. Percy expects to be punished, but is shocked when the teacher turns into a winged Fury and attacks him. The other chaperone, Mr. Brunner, appears and throws Percy a pen that changes into a sword. Percy swings at the monster, and she disappears. The other students do not remember seeing anything that happened, and they do not remember Mrs. Dodds. Percy convinces himself it was all a hallucination, except that Grover acts strangely around him, and he overhears Grover and Mr. Brunner talking about him.
As the summer starts, Percy and his mother, Sally, go to the beach. There’s a big storm, and the two are woken up by Grover, who reveals he is not a human teenager, but a Satyr with goat legs. He tells Percy and Sally they are in danger, and the three drive to a mysterious summer camp. Suddenly, they’re attacked by a Minotaur, who knocks Grover unconscious and grabs Sally, who disappears in a flash of gold light. Thinking his mother is dead, Percy attacks the Minotaur and kills it, then retreats into a nearby farmhouse. He wakes up three days later and finds he is at a place called Camp Half-Blood, a retreat for young demigods — and he is one of them.
Percy meets many other young demigods at the camp, including Luke Castellan, the son of Hermes; Annabeth Chase, the daughter of Athena, and Clarisse La Rue, daughter of Ares. Percy’s powers continue to emerge, indicating he has links to the water. When a Hellhound sneaks into the camp and attacks him, he is protected and publicly claimed by the God of the seas, Poseidon. Mr. Brunner, who is actually Chiron the centaur, runs the camp; he summons Percy to tell him about a pact between the three elder Gods, Poseidon, Zeus, and Hades. They swore not to have any more children, and Percy is a violation of that oath. In addition, Zeus’s lightning bolt has been stolen, bringing the old Gods to the brink of war. Percy is tasked with finding the bolt, and asks Annabeth and Grover to accompany him on his quest.
They travel to the realm of Hades, whom Chiron thinks is the most likely culprit. They fight several mythical beasts along the way, but when they track Hades to LA, they find out that his Helm of Darkness, his equivalent symbol of power, has been taken as well. Hades accuses Percy of stealing the Helm and reveals that Percy’s mother is his hostage. He threatens to kill her, but they escape and discover that Ares has been manipulating them and is the actual thief. Percy defeats Ares in a duel. Hades, realizing that Percy wasn’t the thief, returns his mother safely to New York.
Percy takes the lightning bolt back to Zeus, who forgives Poseidon’s breaking of the oath and lets Percy go safely. He returns to camp and has fun for the rest of the summer. On the last day, he and Luke Castellan are hanging out, when Luke suddenly turns on him and attacks him. Luke reveals that he is the true thief, and both he and Ares were working for the same villain — Kronos, the Lord of the Titans and the predecessor to the reign of Zeus. Percy is shocked, but Luke explains his belief that the Gods must be overthrown because they’re poor leaders. He says it is time for the demigods to rise, and offers Percy the chance to join him. Percy refuses, and Luke attempts to kill him with a poisonous scorpion. Percy manages to kill the dangerous arachnid, but is poisoned in the process. He is in a coma for several days while Luke escapes, and when he wakes up he is given the choice to stay at the camp full-time with a lot of the other demigods, or return home to live with his mother for the school year. He chooses to go home—even though it will be more dangerous, he feels that he belongs in both worlds.