Kuroko Koumori is a mass murderer who killed 715 people, starting in childhood, and was sentenced to death. To combat the rampant crime overtaking her hometown, the government indefinitely stays her execution and recruits her as a state-sanctioned executioner under an extraordinary legal measure. By her side is Hinako Tozakura, a seemingly carefree young woman who acts as both partner and watchdog, possessing unparalleled driving skills that let her perform vehicle stunts defying physics and gravity. Together, this unlikely duo is tasked with hunting down and eliminating the most dangerous criminals that the police cannot apprehend.

The setting is a modern Japan overrun with violent crime, where the legal system has been pushed to its limits. Kuroko and Hinako operate in a gray zone between law and vigilantism, facing heinous offenses such as serial killings, organized crime, and domestic terror. Kuroko is a tall, lascivious lesbian with razor-sharp instincts and a chilling aptitude for murder, but she also displays a mischievous charm and domestic skill that contrast with her brutality. Hinako, whom Kuroko affectionately calls Kuu-chan, navigates their missions with a happy-go-lucky attitude, driving Ferraris without regard for traffic rules. They are joined by Chiyo Yanaoka, the daughter of a notorious yakuza boss, and later encounter allies such as the underground fighter Sabiura Ran and the sniper Reiko Kuchiba.

The narrative unfolds in a series of arcs that escalate from isolated killer hunts to a full-scale war against the extremist organization Sakura Senkai, led by the escaped convict Higaki Tokishiro, who was once an executive of the ultra-violent WWII-era group Senjin no Shishi. This conflict culminates in a chemical-weapon terrorist attack on the Metropolitan Police Department, forcing Kuroko, Hinako, and their allies to use every resource at their disposal. Additional storylines involve a vacation interrupted by a giant robot chasing the scientist Tokiwa Saki, an old acquaintance of Kuroko's. The series blends stylish crime action with dark humor, erotic undertones, and grotesque violence, all while exploring the dynamic between a remorseless killer and her morally grounded—though equally unconventional—partner.