Riena, a serious girl of marriageable age, was worried because she couldn't keep up with her classmates' conversations about sex and had no one to talk to about it. One day, she decided to go to her homeroom teacher's house on a holiday to ask him to teach her about sex. However, the teacher was a genuine lustful teacher who visited a brothel every Sunday. When a student came to his house to visit a brothel he had reserved, he tried to get rid of her, but she was so persistent that he took Kokoro of her curiosity and played with her body, which was still in the process of development. Riena also took what the teacher said seriously, and gradually her body was trained to do as the teacher pleased...