The story takes place in the Masked Ballroom, a unique district that functions as a government-approved slum within a suffocating surveillance state. It is a place where people are encouraged to abandon reason and revert to their primal instincts, finding freedom in anonymity. This chaotic environment is made possible by a monster supplement, a substance that allows anyone to temporarily transform into beastly forms and do as they please.

Patrolling the dangerous streets of the Masked Ballroom are Reiji, a young vampire who is considered a true monster, and his partner Tsuki, a werewolf. Together, they work as cleaners, tasked with keeping the peace and disposing of the city’s many hazards. Their duties range from tracking down centaurs who commit hit-and-run crimes to fighting life-and-death battles against urban legends that have manifested into reality.

Their routine work takes a dramatic turn with the appearance of a mysterious figure known as the Medicine Witch. This meeting sets Reiji and his friends on a path that will change their fates forever. In the second volume, the focus sharpens on the dangerous distribution of an unstable variant of the monster tonic. The pharmaceutical company Beast Tech is responsible for distributing the Monster Tonic, but a dangerous offshoot called Mythic Tonic has leaked into the slum. Reiji, Tsuki, and a new teammate named Kei are hired by their classmate, Mei, to track down this deadly substance after it leads to the death of her friend. As they delve deeper into the investigation, an unprecedented tragedy unfolds, and the reported death count following the incident tragically includes three high school students.