Reiko Fujikura is 45 years old and has been living alone since divorcing her husband five years ago. Her son Hiroshi has recently become a high school student, reaching an age where he might soon have a girlfriend. One day, as she was about to enter the bathroom, she discovered her son secretly masturbating inside. The shock was overwhelming—her mind went completely blank, and the image remained burned into her thoughts. From that moment on, she couldn't stop thinking about her son's masturbation, and eventually, she too found herself engaging in the act.