The story centers on a man named Train Heartnet who withdrew from an elite group of assassins called the Toki no Bannin (時の番人; the Chronos Numbers) to become a Sweeper (a bounty hunter).
In Train's world, there is a secret organization, Chronos, that rules one third of the world's economy through underground control of almost every kind of business. To maintain the balance of the world, Chronos is essential, and therefore, there is the need to have people to protect Chronos. Besides all normal erasers, there are special elite assassins with Orihalcon-made weapons called the Chrono Numbers. Chrono Numbers have their numbers engraved on their body and their weapon. There were twelve total Chrono Numbers, but when Train Heartnet, an unsurpassable shooter, decided to join Chronos in their cause, the number XIII was founded. No one that was targeted by Train Heartnet, also known as Black Cat, for bringing "bad luck", and his extremely quick movements and attacks, ever survived. The name Black Cat was the most feared among assassins.