Despite her appearance as a brown-haired gal, Yuzu, a high school girl who hasn't yet experienced her first love, ends up living in the same room as Mei, who has become her stepsister due to her mother's remarriage. Although Mei is the student council president and an honor student, she unexpectedly initiates kisses, leaving Yuzu completely bewildered. Yuzu spends her days unable to take a step forward, held back by the reason of being 'sisters.' In the midst of this, her childhood friend Matsuri suddenly pushes her down. Unbeknownst to Yuzu, Mei witnesses the scene where she is forcibly kissed by the now dark-hearted, trendy middle school character Matsuri. Jealous of their sisterly relationship, Matsuri then turns her aggressive intentions towards Mei...!?