In Tokyo, a down-on-his-luck American nicknamed Ladybug is tasked with his latest assignment: to retrieve a briefcase aboard a high-speed train heading from Tokyo to Kyoto. Unbeknownst to Ladybug, multiple other assassins with interconnected but conflicting objectives are also aboard the same train. Ladybug aims to carry out his mission peacefully following advice from his new-agey handler, intending to avoid the violence that has marked his past assignments. Nevertheless, his plans quickly derail as he encounters an array of bizarre and dangerous characters.

Among the passengers is a pair of British hitmen known as Tangerine and Lemon, who share a brotherly bond and a professional ruthlessness. They are in charge of transporting the son of a Russian mobster, ensuring the boy's safe return to his father. Meanwhile, an enigmatic schoolgirl with a penchant for manipulation plays her own game, her motives shrouded in mystery. Each character's mission is gradually revealed through their interactions, with the confines of the speeding bullet train forcing them into a series of escalating confrontations. As the journey progresses, alliances are tested, secrets are exposed, and the fight for survival becomes as fast-paced as the train itself.