This spring, at the local prep school Seiyou Academy, where the protagonist (nameable) attends. He has something that he cannot share with anyone, a secret.
"Today is a good day for the entrance ceremony, to all new students. Congratulations on your admissions."
"On behalf of the current students, I sincerely welcome you."
Smiling on the podium, the student council president Yukinoshita Hotaru, due to the recent remarriage of their parents, she has become the protagonist's step-sister.
A person he admires, since he has feelings for her, he keeps his distance. Because his sister thinks that he is adorable and likes him, she is unable to get closer to him.
The turning point of their relationship comes, when suddenly, their parents decide to go on a trip. Naturally, the protagonist and his sister are left to take care of the house. Whether they like it or not, their feelings risen.
A secret cohabitation life, now, it begins!