Yuzuko Aihara, a high school girl whose main interests are fashion, friends, and having fun, is about to get a reality check. Due to her mum's remarriage, Yuzu has transferred to a new, all-girls school that is extremely strict. Her real education is about to begin.

From Day One, happy-go-lucky Yuzu makes enemies, namely the beautiful yet stern Student Council President Mei. So what happens when a dejected Yuzu returns home and discovers the shock of her life: that Mei is actually her new step-sister who has come to live with her? Even more surprising, when Mei catches Yuzu off guard and kisses her out of the blue, what does it all mean?