The main character of this game is a detective named Ryuuichi. He and his assistant, Mizuki, were on a way to a sanatorium, a place of remedy for mental illness patients, for investigation as he received a mystery slip asking to save her before next full moon. Upon the arrival of this sanatorium, he realized something strange about this place: everything is too calm. As he begins to investigate, a horrible scheme will survive little by little until the next full moon.

[Edited from animedensetsu]