In a world where committing a crime can unlock a supernatural ability known as Sin, society faces an impossible crisis. Only those who break the law gain extraordinary powers, creating a landscape where the most dangerous criminals are also the most powerful. While ordinary police officers struggle against flames, telekinesis, and other lethal abilities, the justice system finds itself on the brink of collapse.

The story follows Ren Hizuki, a skilled detective with the Tokyo Metropolitan Police's Investigation Division 1. Despite having no abilities of her own, she battles Sin-wielding criminals daily through sheer physical prowess and determination. Her relentless pursuit of justice leads to an unexpected reassignment to a newly formed unit: the Public Security Bureau, Division 9 - Special Abilities Countermeasure.

Division 9 operates on a dark and controversial principle: fighting fire with fire. The unit pairs non-powered detectives with convicted criminals who possess powerful Sins, essentially turning prisoners into assets. These criminal partners, referred to as pets, are controlled by their detective handlers through strict protocols and absolute command authority.

Upon joining Division 9, Ren is partnered with Yushiro Oboro, a detective who initially appears cowardly and reluctant to engage in combat. His timid demeanor hides a shocking truth. Oboro is not merely a cautious officer but a condemned death-row inmate with devastating supernatural abilities. The partnership between a justice-driven detective and a convicted criminal serves as the emotional core of the series. Their relationship is not a simple friendship but a precarious bond between owner and pet, gradually evolving into a complex interdependence that blurs the lines between captor and captive, justice and vengeance.

The manga explores how Division 9 manages its dangerous assets. Handlers possess absolute command authority over their pets, with the power to authorize immediate termination if a criminal goes out of control. The use of abilities exacts a heavy toll on users, eroding their sanity and damaging their bodies, which adds a layer of desperation to every confrontation. Ren herself struggles with a nicotine dependence that ties into her combat effectiveness, highlighting the personal costs even non-powered officers pay.

Early narrative arcs focus on Ren and Oboro learning to work together despite their mutual distrust. Ren must reconcile her sense of justice with using a criminal as a weapon, while Oboro must decide whether to see Ren as merely another jailer. As they investigate cases involving other Sin users, the hidden agenda behind Division 9 formation and Oboro true past gradually surface. The unit operates in the shadows, with classified activities unknown to the general public and even to most of the National Diet. The story follows their investigation of ability-related crimes across Tokyo, including locations like Akihabara, while higher powers within the government watch and manipulate from behind the scenes. Each case forces Ren and Oboro deeper into a conspiracy that questions whether using evil to fight evil can ever lead to true justice.