Rei, the deadly but reluctant assassin for the Zero Division of the Tokyo Police Department, befriends a young man after she sees him cruising a gay bar looking for johns. After he gets beaten up by a group of street thugs, she takes him in to live with her. When a series of vicious murders takes place, she begins to suspect that the killer may actually be her friend taking revenge on everyone who has abused him. Will she uphold her duty to the police, or protect her only friend?