On death row, a serial killer who has long been tormented by his own uncontrollable murderous urges awaits execution. He greets his death sentence not with fear, but with relief, hoping it will finally put an end to his violent compulsions. This peace is short-lived. He awakens in a fantasy world, reincarnated by a goddess who has a terrible problem on her hands.

The goddess admits to a grave error in judgment. Previously, she selected twelve heroes from another world and granted them immense cheat abilities to slay the Demon King. These champions succeeded in their mission, but victory corrupted them. The twelve reincarnated individuals have since become far worse monsters than the Demon King they defeated, using their powers to oppress the world. Knowing she cannot stop them with divine power alone, the goddess has devised a dark solution: to kill a sinner, she must send another sinner. The newly reincarnated serial killer is given a single mission: hunt down and eliminate the twelve rogue heroes.

The protagonist is reborn under the name Shirukyi. He retains no memory of any special combat magic or overpowered gifts. Unlike the previous champions, the goddess does not grant him any cheat abilities. Instead, his only tools are the detailed knowledge she provides on how each of the twelve heroes operates, combined with his own intrinsic understanding of human depravity and his honed skills as a killer. The narrative follows Shirukyi as he is thrust into a tense game of cat and mouse. He must methodically stalk, outsmart, and assassinate targets who possess godlike powers, all while grappling with the bitter irony that the very murderous impulses he wished to escape are now the only thing keeping him alive and serving a divine purpose. The series explores the clash between the brutal pragmatism of a criminal mind and the high-fantasy setting of heroes and magic, creating a suspenseful twist on the traditional isekai genre where the protagonist is not a savior, but a necessary evil sent to clean up the goddesss failed experiment.