Mamoru Akutagawa, a demon-turned-human, learns that he can turn back into a demon by using the soul of his twin sister, Kurumi. Mamoru, who knows how to do it but cannot do it right away, protects Kurumi, who is involved in misfortunes at every turn, and circumvents the watchful eye of his former angel brother, Kyoyo. Former demon x former angel x unhappy girl, a new slapstick brother-sister comedy begins!