Arata Shiunji has spent his life looking out for his six younger siblings, five sisters and a brother, in Tokyo's Seijou neighborhood, never once dating because family came first. When the youngest, Kotono, turns fifteen, their father tells them the truth: only twins Minami and Shion share blood, and the rest of the seven were all adopted separately into the same household. The revelation upends how Arata sees siblings he's protected for years, as feelings none of them expected start surfacing now that blood is no longer the reason they're a family.