At Lillian Girls’ Academy, a renowned all-girls Catholic school, pairs of girls become soeurs, or “sisters”, in order to support and guide each other during their time at school. The shy Yumi Fukuzawa thought she was far too ordinary to attract a soeur as popular and beautiful as Sachiko Ogasawara, but for some reason, Sachiko has set her sights on Yumi — but why does she want to become Yumi’s soeur, anyway, and just what will become Sachiko’s soeur entail?