Naoto Rindo is the only heir to a family that became rich after World War II. Each Rindo generation has a tradition that is believed to bring wealth to the family. In a month's time, Naoto must undergo a rite known as Hinotsukisai. But this requires Naoto's consort to grant him an heir, or at least to be pregnant. Unfortunately, right after the wedding his beloved woman tragically died. Naoto's grandfather forced his grandson into a relationship with another sister of the family, Mizuko, who owed a lot to the rich clan and practically existed on its money. But, as luck would have it, despite having sex with Yuiko Mizuki every day, the girl did not get pregnant. The head of the clan gave Naoto one last chance. In order not to be expelled, he has to live in Mizuki's house and impregnate any (at his choice) of Yuiko's three younger sisters. Except that the girls are not very happy about such a prospect. Time is short, and Naoto is forced to hurry things up...