At the pinnacle of the Empire State Building, Zeus (Sean Bean), the ruler of the sky and the god of thunder, confronts his brother Poseidon (Kevin McKidd), accusing Poseidon's son, Percy Jackson (Logan Lerman), of having stolen Zeus' master lightning bolt. Poseidon defends Percy, pointing out that he remains unaware of his divine heritage, but Zeus warns that if the bolt is not returned to Mount Olympus by midnight on the Summer Solstice—just two weeks away—a catastrophic war amongst the gods will ensue.
The story follows the struggles of sixteen-year-old Percy, who grapples with dyslexia and ADHD yet possesses an extraordinary skill for underwater survival. During a school trip, he faces an attack from Alecto (Maria Olsen), a Fury disguised as his substitute English teacher, who demands the lightning bolt. Fortunately, his loyal friend Grover Underwood (Brandon T. Jackson) and his enigmatic Latin teacher Mr. Brunner (Pierce Brosnan), a centaur guarding the secrets of the demigod world, come to his rescue. Mr. Brunner provides Percy with a pen that transforms into a powerful weapon and instructs Grover to escort Percy and his mother, Sally (Catherine Keener), to Camp Half-Blood—a secretive haven on Long Island for children of the gods. Tragically, during their journey, they encounter the Minotaur, who appears to take Percy's mother away. Displaying courage, Percy uses the magical pen to defeat the creature by utilizing its own weapon against it.
After waking up three days later, Percy's world is turned upside down when he discovers his true identity as the son of Poseidon—his learning difficulties stemming from a brain meant for ancient Greek, and his ADHD reflecting his innate energy and readiness for action. Grover is revealed to be a protective satyr, while Mr. Brunner is actually Chiron, a noble centaur with a storied past. Chiron emphasizes the urgent need to retrieve the missing lightning bolt before further doom befalls the Earth, as the gods would undoubtedly take sides, leading to vast destruction.
As Percy trains to harness his demigod powers—ranging from manipulating water to healing—he meets Annabeth Chase (Alexandra Daddario), a daughter of Athena, and Luke Castellan (Jake Abel), who is the son of Hermes. He soon encounters an ominous vision of Hades (Steve Coogan), who reveals that Percy’s mother has been captured and taken to the Underworld. In defiance of Chiron's guidance, Percy decides to rescue his mother, taking Grover and Annabeth along for the perilous journey.
With stolen winged shoes from Hermes and a map leading to precious pearls owned by Persephone (Rosario Dawson), Hades’ imprisoned wife, Percy embarks on this daring adventure. He fights and decapitates Medusa ([Uma Thurman]) to retrieve the first pearl, while the second requires a treacherous leap into a statue at the Parthenon in Nashville. Their detour to the Lotus Hotel and Casino in Las Vegas almost claims their mission, as they become entranced by the lotus flowers and lose track of time—until Percy's connection with Poseidon snaps them back to reality.
Once they acquire all three pearls, the trio ventures into the Underworld, where Hades confronts them and reveals that the lightning bolt was hidden in Luke's shield all along. Just as they are in peril, Persephone intervenes, angry at Hades for his treatment of her, and gifts the bolt to Percy. However, with limited pearls for escape, Grover bravely stays behind, allowing Percy, Annabeth, and Sally to teleport to the Empire State Building.
In a shocking twist, they are confronted by Luke, who unveils his meticulous plan to overthrow the gods, claiming he seized the bolt to dismantle Mount Olympus. An intense battle ensues across Manhattan, ending with Percy triumphantly reclaiming the bolt for Zeus and reconciling with his father, Poseidon.
In an engaging mid-credits scene, Sally takes a stand against her abusive husband Gabe Ugliano, kicking him out of their home. However, in a twist of fate, he attempts to retrieve a beer only to meet a petrified end at the hands of Medusa’s head.
