Lillian Academy`s Student Council is called the Mountain Lily Council. The Council is composed of three girls, each of whom takes a flower title: Rosa Gigantea (white); Rosa Sinensis (red); Rosa Foetida (yellow).
Each Rosa can take a junior, referred to as "Rosa ... en bouton" [burgeoning rose]. Each "en bouton" is allowed to pick a junior, referred to as "Rosa ... en bouton petite soeur." The usual pattern is a senior student as Rosa, a second-year student as "en bouton" and a first-year student as "petite soeur" [little sister].

The heroine of the story, Fukuzawa Yumi, is a cute first-year student at Lillian Academy. She has always admired Sachiko (the "Rosa Sinensis en bouton") but when Sachiko asks her to become her "petite soeur", Yumi declines.
When pressed for a reason, she herself isn`t sure, but she knows that something is not right...