Sumire is unsatisfied with the pleasure she gets from sex with her husband and often resorts to masturbation to relieve herself. One day, she accidentally discovers her son, Shinpei, masturbating in the bathroom. After that moment, every time she recalls the image of his erect crotch, she feels her own arousal growing uncontrollably. Unable to suppress these feelings any longer, a new and forbidden shift begins in her relationship with Shinpei!