After your father remarries, you have to live as siblings in the same house with the most beautiful girl in the grade named Ayase Saki.

Now that she is your step-sister, both of you are aware of the relationship between a boy and a girl so you both promise not to go into each other's personal lives and keep a distance from each other.

One day, she wear her uniform to go to school, then she look at you.

"Let's act like strangers to each other, okay?"