Train Heartnet is a famous assassin known as Black Cat. He works for Chronos, an organization that secretly controls the world. However, he begins to question the morality of his actions after he befriends two bounty hunters named Saya and Sven, a thief named Rinslet, and a young girl named Eve.
The bounty hunter Sven is barely scraping by when he crosses paths with the Black Cat (a.k.a. Train Heartnet) and the young bio-weapon Eve. The three new companions will need more than luck to survive when they find themselves sought by both the Chronos Numbers and a Taoist revolutionary group called the Apostles of the Star.