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.