Iroha, a serious girl of marriageable age, was secretly troubled by not being able to keep up with her classmates' conversations about sex. One day, she decided to go to her homeroom PE 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. At first, he tried to turn away Miiro, who had come to his house to visit a brothel he had reserved, 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. Miiro also took what the teacher said seriously, and gradually her body was trained to do as the teacher pleased...