Kojou Akatsuki's life as a regular high school student in the Demon District of Itogami Island ends suddenly after a fateful encounter grants him the extraordinary powers of a vampire. Before long, he becomes the center of attention when it is revealed that he is the fourth Primogenitor, an immensely powerful vampire whom most believe to be nothing more than a legend. Fearing Kojou's destructive potential, the Lion King Organization dispatches an apprentice sword-shaman, Yukina Himeragi, to monitor him and, should he become a threat, kill the boy regarded as the world's most powerful vampire. Forced together by circumstance, the two form an unlikely alliance as Kojou adjusts to his abilities and they both strive to protect the city from various emerging chaotic forces.