At the private Lilian Girls' Academy, where purebred maidens gather, there exists a system known as "sisters" (スール) in the upper grades to carry on the tradition of a pure and righteous school life. When a ritual of exchanging rosaries is performed, the vow to become sisters is made, and the older sister, a senior, guides her younger sister, a junior. Yumi, who has just advanced to the upper grades and has yet to have an older sister, is suddenly declared a "sister" by Sachiko, a second-year student whom she admires as the "Budding Red Rose" (ロサ・キネンシス・アン・ブゥトン). What will happen next?