39-year-old housewife Yasuoka Tamaki lives alone while her husband is away for work. One day, her son Hiroshi sneaks into the changing room and begins masturbating while wearing his mother's large M-cup bra. Tamaki accidentally discovers him in the act, forcing her to confront long-suppressed feelings, and their relationship quickly transforms. Eventually, Hiroshi expresses his desires by embracing his mother's voluptuous body. What begins as an ordinary moment within daily family life spirals into a new, intense emotional connection between mother and son.