Kojou Akatsuki's days as an ordinary high school student in the Demon District of Itogami Island come to an abrupt end after a fateful encounter leaves him with the remarkable abilities of a vampire. It isn't long before he is thrust into the center of attention when it is discovered that he is the fourth primogenitor, an immensely powerful vampire whom most consider to be merely a legend. Fearing Kojou's destructive potential, the Lion King Organization sends in an apprentice sword-shaman, Yukina Himeragi, to monitor, and should he become a threat, kill the boy deemed the world's most powerful vampire. Forced together by circumstance, the two form an unlikely alliance as Kojou comes to terms with his abilities and they both struggle to protect the city from various emerging chaotic forces. The Fourth Primogenitor—That’s the world’s strongest vampire that should only exist in legends. Accompanied by twelve Kenjuu and spreading calamity, this phantom vampire appeared in Japan. For the observation and obliteration of this Fourth Primogenitor, the government and Lion King decided to dispatch an attack mage known as a Sword Shaman. However, for some reason, the one chosen for the observation was a apprentice Sword Shaman girl, Himegari Yukina. Wielding the strongest anti-primogenitor spirit spear, Yukina arrived at Demon District, Itogami City. What is the true identity of the Fourth Primogenitor, Akatsuki Kojou, she encountered over there!?