Housewife Kozue lives with her husband and their college-student son. One day, her husband's sister, Reiko, suddenly appears in their home. Reiko is secretly having an affair with Kozue's husband, and what's more, she gleefully spies on them during sex, watching with a smile as they make love. Her words cut deep: "Such a cute face, yet so dirty, Kozue." Confused and disturbed by Reiko's behavior, Kozue grows increasingly anxious—especially about whether Reiko will set her sights on her son next. As tensions rise, the twisted story spirals further into forbidden desire.