Train Heartnet, dubbed "Black Cat" in the underworld where he carries out killings for Chronos, is infamous for his unfeeling approach to his work. He is known for his deadly precision and utter disregard for human life. One unremarkable evening, Train crosses paths with the enigmatic bounty hunter, Saya Minatsuki. Despite their differences, they form an unlikely bond, and Saya's positive attitude starts to influence Train's outlook on life. No longer content to be a heartless assassin, Train resolves to become a just and fair bounty hunter. His decision, however, doesn't sit well with Creed Diskenth - Train's former subordinate - or Chronos, and both parties will do anything to bring him back into the fold. Now a "stray cat," Train realizes that he can only wander so far before the sound of gunfire catches up with him.