Nanako's husband is away on a work assignment, so she satisfies her desires through nightly masturbation. Wanting to relieve her stress, she decides to start exercising—just as her stepson, Kazuo, gifts her fitness equipment. He personally demonstrates each exercise, closely guiding her movements. In truth, Kazuo has noticed his mother's dissatisfaction. As his hands continue touching her body, he slowly slides them deeper into her folds, beginning a new, forbidden relationship.