One day, Thirua, a devil, turns Yuzuki, the main character, into a girl. He'll need to get back to a man in one month. If he fails, he'll never return to a man. Otome, his senior, knows it, and she takes him to her school dormitory. To get back to a man, he'll need to give Thirua some kind of sex energy every week. Yuzuki tries to save the energy in cooperation with the girls....