Yumi Fukuzawa, a newly-enrolled first year student at the all-girls Catholic institution of Lillian Academy, prefers to keep a low profile. However, as per school tradition, senior students are assigned a junior mentee or "soeur". Yumi finds herself plunged into the realm of the Yamayurikai and their mentees when, after meeting Sachiko by chance, she agrees to become her soeur in a moment of spontaneity.