Sven is your run-of-the-mill sweeper (a.k.a. Bounty Hunter) – down on his luck, haunted by the perpetual grumbling of his stomach and looking to make enough cash just to get by. When a damsel in distress enlists his aid, Sven crosses paths with the worst possible luck – Black Cat (a.k.a.
Train Heartnet). At odds now with the branded assassin, Sven seeks to save a young girl before the unlucky Number can carry out his mission.
Train Heartnet is the infamous gunman ‘Black Cat’, a cold-blooded assassin who works for the organization known as ‘Chronos’, bearing the mark XIII on his body. Most of his life was surrounded by violence, so Train is quiet, asocial and uncaring. However his life changes as he encounters the mysterious Sweeper (bounty hunter), Saya. Her unique way of living interested Train but tragedy occurs, resulting in Train changing the way he has lived towards the life of a Sweeper with newfound friends Sven and Eve.
An organization known to the dark side of the world as Chronos claims to desire world peace, employing the top thirteen assassins in the world known as ‘Erasers.’ They each have a weapon customized to fit their style, all made out of orichalcum ore, the strongest material available on Earth. Number 13, Train Heartnet (codenamed Black Cat) is the most famous and revered of the thirteen Chronos numbers, but in the light of a tragic event he has come to question his path in life. Together with Sven Vollfied, a struggling bounty hunter and a living weapon named Eve, Train takes up a job as a bounty hunter – all the while running from the other Erasers and Creed Diskenth, a crazed man whom wants him to join the Apostles of the Stars, a group made to destroy Chronos.