Train Heartnet, known as Black Cat, and his title as Number XIII in the secret society Chronos. He is a master gunman and a ruthless killer, sent out on missions to assassinate people Chronos deems a threat to them. He continues this life, until one night he meets Minatsuki Saya, a bounty hunter, or more commonly called a Sweeper. She teaches him an invaluable lesson. Train decides to repent for his crimes by living, and he leaves Chronos. Now Train travels with Eve, a mysterious young girl, and his new partner, Sven Volfied. Together they work as sweepers, capturing people who have committed crimes in exchange for money, and facing off against those who want the world destroyed.
