His wife, who is about to give birth, has returned to her parents' home. Keita, who has been enjoying being alone for the first time in a while, reminisces about his single days and openly searches for erotic images in the living room. Then... "Working until late is tough, isn't it?♪" he hears a sudden voice. When he turns around, he sees his wife's older sister , Miyuki. Keita hurriedly hides his crotch. However, Miyuki says something shocking to him: "You were doing it alone just now... so next time, let's do it together." With only one week left until the due date, Keita's long but short life of living together with his wife's overly erotic sister has begun.