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.