Kojou Akatsuki's ordinary life in Itogami Island's Demon District ends abruptly when a fateful encounter grants him vampire powers. As the fourth primogenitor, a legendary and immensely powerful vampire, he attracts attention. To control his destructive potential, the Lion King Organization sends apprentice sword-shaman Yukina Himeragi to monitor and eliminate him if necessary. Forced together, they form an unlikely alliance, protecting the city from emerging chaos.