"One Piece Stampede" is a feature-length anime film set within the expansive world of Eiichiro Oda's "One Piece." The story takes place during the Pirate Expo, a grand festival hosted by Buena Festa, a flamboyant and enigmatic master of ceremonies. The event attracts pirates from across the seas, including the Straw Hat Pirates, rival crews, and even former members of the Roger Pirates. The Expo is centered around a competition to find the legendary treasure of Gol D. Roger, hidden on the island hosting the festivities.

The Straw Hat crew, led by Monkey D. Luffy, joins the chaotic race for the treasure, encountering formidable opponents such as Douglas Bullet, a former member of the Roger Pirates with immense strength and a grudge against the world. Bullet seeks to claim the treasure to assert his dominance and challenge the world order. As the competition intensifies, alliances form and dissolve, with pirates and Marines alike clashing in a series of large-scale battles.

Key characters from the "One Piece" universe play significant roles, including Trafalgar Law, Buggy the Clown, Smoker, Sabo, and Hancock, among others. The Marines, led by Admiral Kizaru and former Admiral Sengoku, also intervene, adding to the escalating conflict. The narrative builds toward a climactic showdown as Luffy and his allies unite to stop Bullet, whose overwhelming power and destructive ambitions threaten to engulf the entire island.

The film emphasizes high-energy action sequences, showcasing the unique abilities of various characters while advancing the overarching themes of camaraderie and ambition central to the "One Piece" series. The story remains accessible to both long-time fans and newcomers, offering a self-contained adventure within the larger "One Piece" narrative.