When Amane Nakanose's mother announces her plans to remarry, Amane eagerly looks forward to gaining a younger brother. However, her excitement wanes when Naoya Akasegawa reveals himself as her 'younger' brother, only being three days younger than her. On the other hand, Naoya's vision of having an older sister is shattered when he meets Amane. Despite Amane's initial coldness and insistence on being called 'onee-chan,' Naoya attempts to adjust to his new life with his unconventional sister. As Amane's tough exterior softens, Naoya discovers a kind-hearted girl beneath it all, sparking a newfound optimism in their unconventional sibling relationship.