The Shiunji family, known for their wealth and status, is made up of seven siblings: Banri, Seiha, Ouka, Arata, Shion, Minami, and Kotono. Arata, the designated heir, is often teased by his sisters for his lack of romantic success. But everything changes on Kotono's 15th birthday, when the siblings uncover a startling, long-concealed truth—every one of them was adopted.
The siblings unanimously decide to keep their family dynamic unchanged. However, this agreement quickly unravels as each sister starts making advances on Arata that go far beyond sibling affection. Arata, who values his family bonds above all else, must find a way to preserve their precious connection and ensure the Shiunji family remains exactly as it has always been.