While trying on her pregnant daughter Reina's old clothes, Minako is mistaken for her daughter by her son-in-law, Akio. This accidental encounter sparks a hidden passion, leading the two to enter into a secret sexual relationship to satisfy Akio's desires while Reina abstains from intimacy during her pregnancy.
Akio and Rena's mother continue their illicit affair, risking discovery even while Rena sleeps beside them. As Rena's pregnancy reaches its climax and she is admitted to the hospital, the pair shares a final, passionate encounter before the baby is born. After a period of normalcy following the birth, the cycle begins anew when Rena becomes pregnant for a second time, leading Akio and the mother to resume their secret relationship.