At Lillian Girls’ Academy, a renowned all‑girls Catholic school, students pair up as soeurs, “sisters”, to support and guide one another throughout their school years. The shy Yumi Fukuzawa believes she is too ordinary to attract a soeur, yet the popular and beautiful Sachiko Ogasawara sets her sights on Yumi. Why does Sachiko want to become Yumi’s soeur, and what will that role entail?