Rei is an agent with no memory of her past, working for a secretive and off-the-books division of the Tokyo police known as the Zero Department. She is assigned to eliminate a group of crime bosses who are tightening their grip on the city. As she carries out her mission, Rei is haunted by fragmented, painful visions of her deceased father.

At the same time, the crime bosses, led by Daidoji, hire a professional hitman named Katsumura to eliminate the mysterious assassin targeting their organization. When Rei and Katsumura cross paths, they do not immediately fight. Instead, they talk and discover a shared reluctance to carry out their orders, as both feel like expendable pawns in a larger scheme. This mutual understanding develops into a dangerous romance, with each finding a reflection of their own isolation in the other.

As their relationship deepens, Katsumura is given a new target: Rei. Pushed by their respective bosses, they are forced to confront where their true loyalties lie, with Rei facing an impossible choice between her duty to the Zero Department and her connection to the man sent to kill her. The story unfolds against a neon-lit, rain-slicked urban landscape, culminating in a final ultimatum that leaves her final decision in doubt until the very end.