Teacher Eromang tells the story of Masamune Izumi, a high school student and light novel writer, who has a sister squatting at home. Izumi Sagiri, who just became his sister a year ago, never left the room. Every day, he stamped his feet on the floor and asked him to help her prepare food. Although this relationship can't be called "brother and sister", it is authentic to find a way to get her out of the room by herself. Because he thinks they are "family" who live alone. One day, however, Authentic suddenly discovered the shocking fact. "This partner" is actually his sister!