Percy Jackson is a seemingly normal boy living in New York City with his mother. After a series of strange events, Percy finds his way to a place called Camp Half Blood where he discovers that he is actually a demigod: a child of a mortal woman and a Greek God.
While at Camp Half Blood, Percy learns that the Greek Gods are actually real and have adapted to Western culture. Furthermore, he learns that demigods are actually rather common. He meets many other demigod children including Annabeth Chase, daughter of Athena, Luke Castellan, son of Hermes, and Clarisse LaRue, daughter of Ares.
Several strange events occur that begin to reveal his powers as a demigod. After an intense match of Capture the Flag and a run-in with an unexpected monster, Percy is finally claimed by his Godly father, and is shown to be the son of Poseidon.
A few days later, Percy is summoned to the Camp headquarters, known as the Big House. It is there that he learns from his mentor Chiron that Zeus's master Lightning Bolt has gone missing and that he is the prime suspect. As it turns out, Zeus, Hades, and Poseidon, being the strongest of the Gods, took an oath to never sire anymore demigod children. Percy, merely by being alive, is a violation of this oath. Thus he was believed to have stolen Zeus's master bolt.
Chiron tasks Percy with finding Zeus's master bolt. Percy chooses Annabeth and his best friend, Grover the satyr, to accompany him on his quest to the Underworld. There he hopes to confront Hades, the likely culprit behind the theft.
From Left to Right: Percy, Annabeth, Grover
The three friends travel across the country from New York to Los Angeles, battling many creatures from Greek mythology including Medusa and the Chimera.
Percy and his friends travel to the Underworld, located in Los Angeles, and manage to confront Hades. Hades informs them that his Helm of Darkness has also been stolen, and he accuses Percy of the theft. (Deja Vu, anyone?) Hades informs Percy that he is holding his mother Sally hostage in the Underworld. And he threatens to kill her and release the dead upon the world of the living if the Helm is not returned to him.
Percy and co. manage to escape the Underworld, and after they do, they realize that Ares, the God of War, has been manipulating them the whole time. Ares is the one in possession of both the Master Bolt and the Helm of Darkness. Percy, tired of being manipulated by the gods, challenges Ares to a duel on the beach. Using his godly powers over the ocean, Percy is able best Ares in the duel, and Ares relinquishes the Master Bolt and the Helm of Darkness to him.
Percy returns the Helm of Darkness to Hades, who fulfills his promise and releases his mother back to the real world. Percy then takes the Master Bolt to Olympus, located at the top of the Empire State Building in New York. He returns the Bolt to Zeus, and Zeus rewards him by sparing his life and allowing him to live despite Poseidon's broken oath.
With their quest finished, Percy and his friends return to Camp Half Blood, heralded as heroes, where they live out the rest of their summer in peace. On the last day of Camp, Percy finds Luke Castellan training alone in the woods. Luke turns on him and reveals himself to be the real culprit behind the theft of both the Master Bolt and the Helm of Darkness.
Luke Castellan, Son of Hermes, and the true Lightning Thief
Luke reveals to Percy that he is acting under the orders of Kronos, the Lord of the Titans. He explains to Percy that the Gods are irresponsible and poor leaders who deserve to be overthrown. He offers Percy the chance to join him, but Percy refuses. Luke then attempts to kill Percy with a poisonous scorpion before fleeing Camp Half Blood for good.
Kronos, as depicted in the Lightning Thief movie
Percy manages to kill the scorpion, but is poisoned in the process and nearly dies. Once he has recovered, he is given the choice to return home to his mother, or stay at Camp year-round. He chooses to return home to his mother despite the dangers he faces while being away from Camp. So ends the story of the Lightning Thief.