On the night of the school festival, Yumi officially became Sachiko's little sister after receiving the rosary. A new chapter begins as she starts her life as the little sister of the Red Rose bud (Rosa Kinen-shisu an Buton), but an unexpected major incident awaits her. Rei Hasekura, who was chosen for this year's "Ideal Sister" award (Best Suuru) along with her little sister Yoshino Shimazu from the Yellow Rose bud (Rosa Fetida an Buton), suddenly dissolves their sisterly relationship! As girls influenced by the two return their rosaries to their own older sisters, chaos ensues throughout the school!