Train Heartnet, also known as "Black Cat," was an infamous assassin for a secret organization called Chronos… until he abandoned that cold-blooded existence to live on his own terms as an easygoing bounty hunter. But is Train's past as far behind him as he thinks?

In the last chapter of Black Cat, Kentaro Yabuki stated that "This is more like the end of part one". and says "If the opportunity present itself, I may begin writing up Black Cat part 2".