The world is secretly controlled by a secret organizition, Chronos. Under the command of this mysterious organization are the Chronos Numbers, a group of powerful assasins, taking orders to eliminate certain targets secretly to keep the so-called peace. One of the most powerful is the Number XIII, Train Heartnet, also known as Black Cat. At first he seems to be a cruel killer, however a meeting with a girl Sweeper changed him completely. Now as a Sweeper, Train travels with his partner Sven Vollfield to hunt down criminals.