48-year-old Rieko Masaki is a full-time housewife supporting a family of five with her husband and three children. Now in her 26th year of marriage, she finds herself unsatisfied with their intimate life. Although they have sex once or twice a week, her husband describes it as a "routine ritual to reaffirm their marital bond," lacking passion or mutual pursuit of pleasure. Rieko realizes the quality of their sex has drastically changed from the early days of their relationship, and she no longer experiences the deep satisfaction or intense pleasure she once knew. She still remembers the shock and ecstasy of her first vaginal orgasm, but that alone no longer fulfills her. Craving true pleasure and seeking new stimulation, her heart begins to lean toward infidelity. Rieko's first affair will become the first step through a new door of pleasure.