Yuichi Mitsui, an ordinary college student, is currently living alone to attend college. One day, his beloved sister visits Yuichi during the summer vacation. His younger sister, who he saw for the first time in two years, has a developed a plump bust of loveliness. Besides her grownup appearence sister, Hana, is still ... a ridiculous idiot.

If she walk three steps, she will forget everything she learned, she can hardly read kanji, and she cannot operate the remote control with three or more switches. Yuichi decides to follow the thorny path in order to raise his younger sister, into a fine adult.