The Lillian Girls' Academy is a renowned Catholic school solely for girls. In order to offer help and direction during their academic journey, girls are paired up as "soeurs" or sisters. Yumi Fukuzawa, feeling unremarkable and apprehensive, never expected to catch the attention of a popular and breathtaking soeur like Sachiko Ogasawara. To her surprise, Sachiko has taken an interest in Yumi. The intentions behind this and the obligations of being Sachiko's soeur, however, remain unclear.