A standard high school romance story with hentai elements. The father of high school student Konda Makoto, who was left without his mother at an early age, is a famous archaeologist. And he is almost constantly outside of Japan, on archaeological digs. But our hero doesn't live in his father's house alone. He is looked after by his father's old acquaintance Narusawa Misako and her daughter Yui. Makoto and Yui have known each other since they were children, and for a long time now Yui has been calling him big brother. But why did she suddenly decide in high school to transfer from the Girls' Gymnasium to Yasuhachi High School? And why does she get so upset every time Makoto supports some girl in trouble?