Sera Fuki and Katou Ui have been childhood friends since they were small children, and they were so close that they would go to each other's rooms through the window. This has not changed since Fuki became a university student and got himself a beautiful girlfriend, Aikawa Seira.

Even though their minds and bodies have matured enough, and they are secretly aware of each other as the opposite sex. Their relationship has never developed into a male-female one.

In particular, Ui has always had feelings for Fuki, but due to her natural shyness, she has never been able to show them off. When Fuki became a university student, he finally got a girlfriend and Ui was about to give up.

However, Fuki coming back home happened to witness Ui changing clothes. Moreover, Ui accidentally witnesses her best friend having sex. Their relationship gradually changes, from the platonic and stagnant relationship they had to one where they fully become aware of one another.

Seira, begins her plan to keep her beloved Fuki with her and to keep the thieving cat, Ui, in check.

[From the official site]