The story takes place at the Lilian private boarding school for girls, founded 1902, proud proud of its long tradition and elegant school style. Surrounded by the rich environment of Musashino, Lilian boarding school owns a kindergarten, a primary school, a middle school, a high school and a college (home economics/literature) and a postgraduate college. The school educates young ladies from noble families and equips them with a rich knowledge and fine manners. alley.

In order to keep the tradition of a clean pure campus, a system called "sisters" has been established in the high school. By exchanging rosaries in a ceremony the "senpai" (seniors) swear to look after and guide the younger sisters, "Kôhai" (the juniors). Yumi just moved on to her first year in high school and has yet never had a "sister" before, when the admired second year student Sachiko suddenly declares that she will be her "sister".