Whenever her husband went on business trips, Natsuko Kayama would bathe alone, unaware that her son Katsu was secretly watching her. The image of her naked body, forever etched in his childhood memories, flushed pink in the steamy bathroom, constantly stirring uncontrollable arousal within him. One day, as Katsu peeked from outside the bathroom again, Natsuko's voice suddenly called out, "Why don't you join me for once?" Obliging her invitation, Katsu stepped into the bath. Overcome with both shame at being caught and overwhelming excitement at the sight of his mother's exposed body, he could barely contain himself. Seeing his distress, Natsuko gently whispered, "Just this once..." and slowly drew him closer, guiding him into forbidden intimacy.