This is a world where humans, demon clans, and fiends coexist. The human and demon races appear harmonious on the surface, but constant conflicts simmer beneath it. The fiends lurk in the shadows, always waiting for an opportunity to launch a massive invasion and seize control of the entire world. Meanwhile, some humans have fallen into depravity, spending their days disrupting society and undermining public order. In response, an organization called the Night Suppression Division was established. Its members appear to come from all walks of life, but whenever night falls, they put on masks and roam the city, silently protecting the peace of the world.