Yumi, a married woman living in Tokyo, had been enjoying peaceful days with her husband, Akira. However, one day, Akira's younger sister, Yuna, runs away from home and takes refuge at the Kazama household. Before Yuna arrived, Yumi and her husband had a harmonious relationship. But after Yuna moves in, an unnaturally close bond begins to form between Akira and Yuna. Consumed by jealousy, Yumi starts treating Yuna harshly. As a result, her relationship with Akira grows colder by the day, while a forbidden connection quietly develops between Yumi and Yuna.