Kobeni Yonomori's life takes a surprising turn on her 16th birthday when she learns of the existence of a fiancé and sister-in-law she was previously unaware of. This revelation is the result of an arrangement made by her now-deceased grandfather, which brings Hakuya Mitsumine and Mashiro into the Yonomori household. As Mikakunin de Shinkoukei unfolds, Kobeni grapples with adapting to these unexpected changes in her life while navigating her connection with Hakuya. However, revelations about the siblings' history have the potential to significantly alter Kobeni's future.