In their second year of marriage, Seigo and Miki plan to use a month of saved semen during her most fertile period. Miki's adult older sister Mina also wants a child, but her husband's ED and demanding job have left their marriage sexless. Determined to get pregnant before her younger sister, Mina arrives on the couple's planned baby-making day in a braless outfit that exposes her nipples, seduces Seigo, and steals every drop he worked so hard to save.