Together with her husband, Rin visits her father-in-law, Eikichi, who has fallen ill. That night, Eikichi sees Rin indulge in masturbation, which leads to an unexpected development... Concerned about the couple's sex life, Eikichi mercilessly caresses his wife in front of her son, saying, "If you take care of me, I'll teach you what sex is!" And her father-in-law's "SEX course" continues to escalate.
