Based on the game by Selen.
A long time ago, the Rindo Clan was cursed by a vengeful spirit so that no male children would ever be born. To counter the curse, every three generations, the Rindo must beget children with a family possessing strong spiritual energy. Ever since that day, the Higetsu family have been important and well-treated servants of the Rindo. It is their duty to mix their bloodlines every three generations with that of the Rindo heir.
Time passed and the practice continues to this day. The current heir is Naoto whose fiancee, Suzune, passed away in an accident. Now, he is required to find a replacement.