Shizuka Tachibana endures the shattering loss of her fiancé, Eiji Kuraza, transformed into a demon by experimental drugs from a pharmaceutical company. This event also costs Shizuka her arm. Driven by this trauma, she resolves to uncover the truth behind his transformation and find resolution. Her search leads her to Tsunami Shijo, an ancient guardian running a demon-exterminating detective agency. She formally requests his aid to locate and eliminate her transformed lover, plunging her into supernatural conflicts.

Shizuka's investigation reveals the pharmaceutical company's connection to Takamiya, a demon leader plotting humanity's enslavement. As events escalate, Shizuka becomes entangled in the larger struggle between the remaining guardians and demons within Neo-Shinjuku. Her personal quest for answers and justice regarding her fiancé and her own sacrifice evolves into direct participation in confronting Takamiya's existential threat. The narrative positions Shizuka as a human catalyst within the ancient conflict, bridging the human experience with the hidden supernatural war.