Shinobu Oshima, 46 years old. She lives with her husband and son. Her husband is busy working, often staying late, and shows little interest in family matters. Her son, now an adult, spends his youth freely and rarely comes home early. As days pass in this routine, she finds herself increasingly alone, growing lonely. To cope with the loneliness, she turns to masturbation, finding comfort in solitary pleasure. To protect her privacy and keep it hidden from others, she always keeps it secret. But one night, her hidden habit is unexpectedly discovered by her son.