Yumi is a first year student at the Lillian School for Girls, an exclusive all-girls Catholic school. The school has a tradition that older girls take a younger girl as their little sister or soeur in order to instruct and look out for them. Though Yumi admires Sachiko, a reserved second year student who is a member of the student council, she has no hopes of being chosen as her soeur. When an innocent encounter leads Sachiko to impulsively ask Yumi to be her soeur, Yumi is suddenly brought into the inner circle of the Roses, who are the members of the student council. With Sachiko at her side, Yumi begins to unravel and understand the relationships between the Roses and their soeurs and figure out her own feelings for Sachiko.