Percy Jackson is a twelve-year-old boy with dyslexia and ADHD, who has been expelled from every school he has ever attended. When his class takes a field trip to the Metropolitan Museum of Art, his pre-algebra teacher Mrs. Dodds transforms into a Fury and attacks him. Percy's Latin teacher Mr. Brunner throws Percy a magic sword named Anaklusmos or the English translation, Riptide, and sends Mrs. Dodds to Tartarus. When Percy come out, he finds that nobody remembers Mrs. Dodds and Mr. Brunner acts same.
When Percy and his mother Sally go to Montauk, his best-friend Grover Underwood comes to tell them to leave immediately. They drive away in a horrible storm, and a Minotaur takes Sally in a blinding flash of light. In anger, Percy kills the Minotaur and takes one of its horns. He wakes up three days later, healed by nectar and ambrosia, at Camp Half-Blood. Grover reveals himself to be a satyr, and Mr. Brunner reveals himself to be Chiron the centaur. As nobody knows who Percy's godly father is, he is placed in the Hermes (the god of messengers, travelers, and thieves) cabin. After several water-related incidents, a trident mark appears above Percy's head, this is revealed to be his father, Poseidon, claiming him.
The Oracle tells Percy to find the stolen master lightning bolt of Zeus, because the god thinks that Percy is to blame for stealing the lightning bolt. After defeating several monsters including Medusa herself, they find Hades, who actually took Sally. Apparently, his helm of darkness (his symbol of power) seems to have been lost too. Hades also blames Percy for stealing his helm and wants to kill Percy and his mother and release all the dead back into the real world. They use magical pearls, a gift from a Myriad, to save themselves from Hades and discover that Ares actually has the bolt. Percy challenges Ares to a duel and wins, to his own surprise. Lord Hades realizes that Percy did not steal his helm of darkness, so he returns Sally safely.
After the bolt is given back to Zeus, Percy's friend, Luke Castellan, is revealed to be the lightning thief and the number one servant of the Lord of the Titans, Kronos. Luke tries to kill Percy by calling on a pit scorpion, but luckily Percy manages to kill it. Unfortunately, Luke had gotten away. The story ends with Percy deciding to spend the school year with Sally.