Hachiro and Karen has been friends since they were children… During the day she is a sweet and timid young girl but every night Karen turns into a demon (oni) and tries to go out and eat someone. Yet when she wakes Karen remembers nothing of the night before. Hachiro has made it his mission to keep her from becoming a murderer. Furthermore were she ever to fall in love she will fully turn into a demon. Even if that means he has has to act rough and mean towards the girl he loves to keep her from falling for him. Its a strange situation.