Rei, a reluctant yet highly skilled assassin working for the secretive Zero Division of the Tokyo Police Department, lives a nomadic and emotionally vacant life. Following her orders from a handler named Mutoh, she moves from one sparse apartment to the next, eliminating targets deemed a threat to society. Her latest mission involves taking down a corrupt lawyer, a task she executes with cold precision.

While frequenting a local bar, Rei notices a troubled young man named Mitsuru, who works as a male prostitute. After witnessing him being severely beaten by a group of thugs, she makes an uncharacteristic decision and takes him into her home to recover. As Mitsuru begins to fill the void in her lonely life, a series of vicious murders plagues the city. The victims are all men who have abused or exploited others, and the killings bear the unmistakable signature of a professional assassin, suggesting the culprit could be a rogue Zero agent.

Rei soon begins to suspect that the gentle young man she has befriended might be the killer exacting revenge on those who have wronged him. As she struggles with her growing emotional attachment to her only friend, she is also tasked by the Zero Division to hunt down and eliminate the mysterious murderer. Caught between her duty to the police and her personal loyalties, Rei must uncover the truth behind the killings before she becomes the next target.