Mitsui Yuichi is an ordinary student. He moved out of his parents' house to go to university and now lives alone. He doesn't have a girlfriend... because he's crazy about his little sister. "If this continues, I will cross the forbidden line" these are the thoughts that made Yuichi heartbrokenly choose a university far away from home to keep his distance from his little sister. However, one day during summer vacation, his beloved little sister shows up at his doorstep. After two years, his younger sister has not only remained as cute as she was, but has also acquired a lush bust, a wasp waist, and a firm ass. Hana is the name of the owner of this body of unearthly beauty. But she has one significant flaw. And that flaw is that... ...is that she's stupid as hell. Just three steps and she's completely forgotten everything she's learned. She can't read most of the hieroglyphics, and she can't use a remote control with more than three buttons at all. Yuichi decides to take the thorny path in order to teach her sister, who would otherwise be barred from normal society.