Long time ago, the Rindo family was cursed by a monster. Since then, new born babies have been girls. To break the curse, they had to marry persons of the Higetsu family who have strong spiritual power, and have to bear a child by the next red moon. Now, the curse befalls to Rindo Naoto. In order to break the curse, he has to beget a child with one of four sisters of the Higetsus. (Source: AnimeNfo)