The story centers on a man named Train Heartnet who withdrew from an elite group of assassins called the Toki no Bannin (時の番人; the Chronos Numbers) to become a Sweeper (a bounty hunter).
In Train's world, there is a secret organization, Chronos, that rules one third of the world's economy through underground control of almost every kind of business. To maintain the balance of the world, Chronos is essential, and therefore, there is the need to have people to protect Chronos. Besides all normal erasers, there are special elite assassins with Orihalcon-made weapons called the Chrono Numbers. Chrono Numbers have their numbers engraved on their body and their weapon. There were twelve total Chrono Numbers, but when Train Heartnet, an unsurpassable shooter, decided to join Chronos in their cause, the number XIII was founded. No one that was targeted by Train Heartnet, also known as Black Cat, for bringing "bad luck", and his extremely quick movements and attacks, ever survived. The name Black Cat was the most feared among assassins.
His only objective was "to be the best". In one of his missions, he had to kill a target, but the target's child was at the scene. Train, recalling his childhood trauma, could not kill the target in front of his child, leading to him being shot by the target's bodyguard.
Eventually, he met Saya Minatsuki, who was a sweeper. Through his contact with Saya, Train changed drastically, a change that culminated with leaving Chronos. However, a man who admired Train in an obsessive way, Creed Diskenth, did not accept that an elite assassin was being changed that way. Creed then set out to kill Saya, while Train was not there. Train reached the scene too late and tried to fight Creed, but he escaped.
Six months later, Train is a Sweeper, traveling with his partners Sven Vollfied and Eve. Their lives as sweepers are about to be changed when the present is invaded by the past: Creed Diskenth, after two years of being away, returns. And he wishes to meet Train, with a proposal to become his partner.
A running gag in the anime is usually for a white cat to appear at a time when a character makes a joke but also appears in different places such as in Eve's crystal ball and on one page of Sven's newspaper in episode 13 and 21. The white cat also appears as a shield used by Train whenever Kyoko wants to kiss him.
曾经暗杀过无数政界要人的机密组织KRONOS前王牌杀手TRAIN,人称“BLACK CAT”,相传由于背叛组织而被杀.可是,2年后,他竟然再度出现,做起了“清道夫”的买卖.他拥有“支配眼”的前国际搜查官SVEN搭档,再加上好财如命的变装高手RINSLET和纳米生体兵器EVE,一同寻找搭档CREED,以了结2年前结下的恩怨……