People become primal beasts, give up rational thought, and celebrate their freedom in anonymity. This is the "Masked City." It is a town where anyone can casually transform into a beast by taking a "monster supplement" and do whatever they want. It is the only government-approved "slum" where one can forget the stifling atmosphere of a completely monitored society. In this town, a vampire boy named Zero, who was a real monster, and his werewolf partner Tsuki take on the job of "exterminating vermin" as street cleaners. They search for hit-and-run centaurs, hunt JK bunnies, and fight to the death against an urban legend that has emerged. An encounter with the "Medicine Witch" will change the destiny of Zero and his friends.