Executing every assignment with ruthless precision, Train Heartnet is a notorious assassin who shows no respect for human life. Operating under the alias "Black Cat" in the underground world, this elite killer works for the powerful secret organization known only as Chronos. On a gloomy night, the blasé gunman encounters Saya Minatsuki, an enigmatic bounty hunter, and soon forms an odd friendship with her. Influenced by Saya's positive outlook on life, Train begins to reconsider his existence. Deciding to abandon his role as the Black Cat, he chooses instead to pursue a virtuous path as an honest bounty hunter. However, Chronos—and particularly Creed Diskenth, Train's possessive subordinate—is not impressed by Train's sudden change of heart and vows to resort to extreme measures to retrieve the emissary of bad luck. This assassin turned "stray cat" can only wander so far before the deafening sound of gunfire rings out.