Yuri, the mother-in-law, runs a bar. Teppei, her son-in-law, is fascinated by the unique sex appeal and sexy body of Yuri, who works at night. One day, Yuri hits her head while cleaning and loses consciousness. Teppei happens to find her and tries to look after her, but he is captivated by her defenseless big breasts and ends up playing a prank on her. The son-in-law's desire to get a taste of Yuri's body accelerates, and he presses the suspicious Yuri further, saying, "Let's see if that sperm is mine...!" The two get caught up in a messy affair that they can never tell their families about...