In Misono private school, there's a strange rumor about a female student becoming sexually free-spirited and causing trouble both inside and outside the school. The rumor got a name Kaori Tsuki after a person who caused a previous incident that led to the school becoming co-educational. It's believed to be a mere creation of some students until the protagonist, Mitsui Toshiki, becomes the target of an evil spirit named Kaori, who wants to break up him and his girlfriend, Tokimori Shiina. The story revolves around the struggle of Toshiki to overcome his dark desires and save his girlfriend while unraveling the mystery of an evil spirit.