At the private Lilian Girls' Academy, a place where purebred daughters from prestigious families gather, there exists a system known as "Soeur" (sisters) for maintaining order in school life. Through a ceremony of exchanging rosaries, students pledge to become sisters, with the older sister (senpai) guiding the younger sister (kohai). When Yumi, who has just entered high school and has yet to have an older sister, unexpectedly receives a sister declaration from Sachiko, the admired "Rosa Kinensis Anbutong" (Pink Rose Bud), her world is turned upside down!