Rin visits her father-in-law Eikichi's home with her husband after he complains of feeling unwell. That night, Eikichi accidentally catches Rin indulging in masturbation alone. Concerned about his son's marital sex life, the father-in-law boldly propositions his daughter-in-law in front of his son, saying, "If you take care of me, I'll teach you the true meaning of sex." Right there, he begins mercilessly fondling her, and the incestuous "sex lesson" from her father-in-law quickly escalates into unexpected developments.