Frustrated by a lack of sex with her husband, Mariko secretly consoles herself during her frustrating days. One day, she finds out that her daughter's husband, Shinji, who lives with her, is also secretly masturbating for the same reason. Mariko is Kokoro by an unusually large cock that she happens to see, and tells herself that she is doing it to release her frustrations on behalf of her daughter. She continues to secretly seduce him at any time, day or night. Shinji is unable to resist the voluptuous charm that emanates from his mother-in-law, and continues to play with his cock over and over again.