After his father remarries, Yuuta suddenly finds himself with a new stepsister: Saki, the greatest beauty at school, and while neither is sure how to act around the other at first, they gradually grow more comfortable living together.
Yuuta Asamura is still figuring out how to interact with his new stepsister, Saki Ayase. But after finding they have more in common than expected, the two reach a comfortable equilibrium. Things start to get out of whack, however, when Saki fails one of her exams and asks for Yuuta's help. Then Yuuta's attractive college-age coworker, Shiori, invites him out on a date, and Saki finds a strange new emotion welling up in her heart... Just what is this feeling?