A clandestine operative known only as Rei, the top assassin for the Tokyo Police Department's secret Zero Division, receives an unusual assignment that departs from her usual mission of liquidation. Instead of simply eliminating a target, she is ordered to protect a potential witness. The target is Masahi Kaneda, the flamboyant and deranged boss of a black-market organ trafficking syndicate responsible for numerous deaths. Rei’s only lead is Kaneda’s disgruntled mistress, Nana, who has agreed to testify against him in exchange for police protection.

Initially hostile toward one another, the stoic, emotionless Rei and the spoiled, talkative Nana are forced into an uneasy alliance. After their safehouse is compromised in a violent gunfight, Rei takes Nana to her own spartan, secluded apartment at the top of an abandoned building. Confined together, their relationship evolves from mutual annoyance to a deep, intimate connection as Nana persistently tries to break through Rei's cold exterior. A significant complication arises when a senior politician, whose daughter desperately needs an illegal transplant, uses his influence to shut down the investigation. Rei’s superiors order her to abandon Nana, leaving the talkative witness without protection and marked for death by Kaneda’s organization, especially since Nana has also stolen a large sum of the crime boss's money.

As Rei’s professional obligations conflict with her developing personal feelings, she must decide whether to follow orders or take on the entire syndicate herself. The narrative culminates in a bloody confrontation with Kaneda, whose depravity extends to bisexuality, cross-dressing, and hints of cannibalism, forcing Rei to revert to her lethal skills to resolve the dangerous game.