Yuki, feeling anxious and lonely due to her husband's infidelity, had been masturbating to comfort herself in his absence while he was working away from home. One day, as she was pleasuring herself with a vibrator, her son Hiroki suddenly appeared beside her. Flustered and panicked, Yuki reached toward her son's crotch. That moment became the trigger—Hiroki, having watched his mother masturbate, instantly grew hard.