A boy is initially overjoyed at the news of him having an older sister, due to his father remarrying to a woman who has a daughter. That is, until he finds out that his new sister is actually his school's strict PE teacher, Shina. With his father going off to live with his new wife, the boy will live with alone with his new sister. In their new life together, the boy finds out that Shina at home is completely different from Shina at work: clumsy, scared of insects, and kind.

Yuu is a healthy high school student who harbors a crush for his Kyouko sensei. He absolutely loves everything about her and after fretting about the subject for some time he decides to confess; to his surprise Kyouko Sensei is also in love with him and agrees to date him. .... The catch is that Yuu doesn't know why Sensei is in love with him, neither he knows her preferences when it comes to how to behave to her partner.... And by the time of when Yuu learns this we have that "Yuumi" is born, now Yuu doesn't know if he should somehow enjoy his new relationship or regret it.