The ending of Percy Jackson The Lightning Thief resolves the missing bolt by exposing Luke as the thief, and it closes with Percy rejoining the world he now belongs to. Full spoilers follow, along with my own reading of the film.
What happens. Percy defeats Luke, returns the lightning bolt to Zeus and avoids a war among the gods. He reconciles with his father Poseidon and rejoins Annabeth and Grover at Camp Half-Blood. Sally, back home, throws Gabe out, and he is turned to stone by Medusa's head. The film closes with Percy's new life as a demigod and leaves the door open for further adventures.
Why the shield matters. In my opinion, the smartest piece of plotting is the shield. Luke hands it over as a gift for the road, and the bolt turns up in it, so the villain is effectively carried along on the quest by the heroes he is deceiving. It could be argued that this explains why Percy is blamed so easily: the real thief was standing right beside him at camp, being helpful.
Why Persephone changes things. In my opinion, the film steers you toward Hades as the likely suspect, and the Underworld scene turns that on its head. Persephone turning on Hades, freeing Sally and handing Percy the bolt is what lets the story close without a battle between gods. As such, the final fight is personal, between Percy and Luke, rather than an Olympian war.
The open door. The last scene points forward, and the next film picks up with Luke poisoning the protective tree at camp. Percy ends this film at peace with Poseidon, but a new deadline arrives in Percy Jackson Sea of Monsters.
What came next. A third film was never made. The story moved to television, where the Disney+ series Percy Jackson and the Olympians is a television adaptation rather than a film. Season 3 adapts The Titan's Curse and premieres November 20, 2026, according to Disney+ press, and you can follow film news on the new Percy Jackson movie page.