Mitsui Yuichi is an ordinary student. He moved out of his parents’ home to go to university and now lives alone. He does not have a girlfriend. because he is crazy about his little sister. “If this continues, I will cross the forbidden line,” these very thoughts made Yuichi choose a university far from home to stay away from his little sister. However, one day during the summer vacation, his favorite sister appears on the doorstep of Yuichi’s apartment. After two years, the little sister not only remained as cute, but also got a lush bust, a wasp waist, and an elastic ass. Hana is the name of the owner of this body of unearthly beauty. But she has one significant disadvantage. And this flaw is that… she is stupid to be outraged. Just three steps and now she has forgotten everything she has learned. She just can’t read most of the hieroglyphs, and she can’t even use the remote control with more than three buttons. Yuichi dares to take the thorny path to teach her sister, who is otherwise ordered to enter a normal society.