Follows Yuta Asamura, whose parent gets remarried when he is in high school and he gains a new beautiful younger stepsister, Saki Ayase. Saki is also the #1 student in her grade. Both of them have some knowledge on male-female relationships through their parents, and in order to not cause discord in the family they agree to be not too confrontational but not too compromising either, with a moderate amount of distance between them. Saki is starved for familial love but repeatedly tries to be alone, and Yuta is bewildered at how to be a proper older brother. They gradually learn to become comfortable with each other.