Kobeni Yonomori, who has just turned sixteen, is shocked to learn she has a fiancée named Hakuya Mitsumine who, along with his little sister Mashiro, come to live with her and her older sister Benio. As Kobeni struggles to remember just who Hakuya is, Mashiro finds herself at the mercy of the siscon tendencies of Benio. Later, both Hakuya and Mashiro transfer into Kobeni's class, with Mashiro disturbed to find Benio has a completely different personality at school.