The Okajima couple are in their 50s. Their sex life has not diminished, and if they feel like it, they can do it in the morning. However, the effects of aging are evident, and wife Keiko finds her husband Munao's erection increasingly unsatisfying year after year. In contrast, Munao's son from his previous marriage, Kouta, is a vigorous 30-year-old man. In fact, Keiko has a physical relationship with this stepson and is captivated by his hardness, which is different from her husband's. It's not that she's tired of her husband. She just wants to be thoroughly penetrated as a woman.
