Mika works as an insurance saleswoman. Although she and her husband are constantly at odds, Mika's body is still in the prime of her life and she secretly engages in "sex for money" without her company's knowledge. There she meets a young man named Kiyoshi, who is more than a decade younger than her, and they have a particularly good physical relationship, so they start meeting frequently. They are in a so-called "sex friends" relationship. One day, Kiyoshi is passionately talking to a friend, saying, "If you're going to have a sex friend, it's got to be a mature woman!" But why?