In a future Japan where a suffocating surveillance state monitors every citizen's action, the only place for true freedom is the Masked Ballroom. This is a government-approved slum, an artificial lawless zone where people can ingest monster supplements to transform into beasts, shed their human identity, and indulge their most primal urges without consequence. Amidst this controlled chaos, the young vampire Reiji and his werewolf partner Tsuki work as street cleaners, tasked with exterminating the dangerous pests that threaten what little order remains. Their targets range from centaurs who commit hit-and-runs to the very urban legends that manifest from the city's collective unconscious. As a real monster in a world of temporary ones, Reiji enforces the underworld's peace. His path crosses with the Medicine Witch, a mysterious figure whose knowledge of potent concoctions and the dark supplement trade makes her both a valuable asset and a formidable target. This meeting sets Reiji and his friends on a collision course with the forces that control the city from the shadows.

The narrative is built around the escalating conflict stemming from a black market auction that floods the Masked Ballroom with dark supplements, resulting in widespread chaos and the creation of unstable chimeras. As Reiji and Tsuki investigate the source of this crisis, their inquiry is abruptly shut down by their superior at Fantasy Cleanup, hinting at a deeper conspiracy. The situation explodes when the CEO of BT Corporation, the entity that holds the formula for the base component of all monster supplements, falls into critical condition. The only person who knows the formula and is named as the next CEO is the Medicine Witch, Kei. This instantly makes her the target of a deadly manhunt across the city, triggering a modern Walpurgisnacht where Eastern and Western magic users clash, dark supplement addicts rampage, and ancient horrors resurface. Reiji finds himself in a desperate race against time to protect Kei, not only from rival corporate and magical factions but from the city's very fabric that has turned against her.

The main cast includes Reiji, a vampire who is a "real monster" in contrast to the supplement-fueled masses, burdened by his nature and the solitude it brings. Tsuki, an original werewolf, serves as his loyal and powerful partner in their dangerous work. Kei, the Medicine Witch, is a pivotal figure whose unique knowledge of supplement creation puts her at the center of a power struggle for control over the city. Antagonistic forces include the human-led corporate entities like BT Corporation and various rogue magical elements seeking to exploit the chaos for their own ends. The setting is the Masked Ballroom, a sprawling, neon-lit dystopian sprawl where the lines between human and beast, reality and fantasy, are constantly blurred. Notable narrative arcs include the initial investigation into the dark supplements, the corporate cover-up that aborts the investigation, and the central flight arc where Reiji and the others must protect Kei from an ever-growing wave of assassins, monsters, and magical phenomena unleashed upon the city.