To arrange the inheritance because of the loss of her mother, Hatou Kei heads for her fathers house in Hemizuka. In the train on the way there, Kei has a mysterious dream of a huge tree and a woman who seems to be lost in sadness. In the land of Hemizuka, Kei learns the secret concealed in her own blood through various meeting with an Oni Slayer, a friend of her late mother, a young girl accompanied by a white fox, and even the mysterious girl from her dreams.

[Edited from Wikipedia]