Sachiko Ogasawara, a brilliant and refined student at Lillian Girls' Academy, hides a troubled personality behind her status as the school idol. She has a deep aversion to men and is revolted by the idea of having to dance with a male student for the upcoming school play. Desiring to step down from her role, she proposes to the Yamayuri Council, Lillian's student council, that she will instead prove her worth by finding a petite sœur—an underclassman who becomes the protegee of her grande sœur.

To the bewilderment of all, Yumi Fukuzawa, the first-year student Sachiko invites to become her petite sœur, refuses the proposal. However, senior Yamayuri Council member Sei Satou proposes a challenge: if Sachiko can convince Yumi to become her petite sœur, she can withdraw from the play. Although Sachiko often struggles to connect with others, she endeavors to do everything in her power to win Yumi over.