Kojou Akatsuki's mundane existence as a regular high school pupil in the Demon District of Itogami Island abruptly ceases when a life-changing encounter imbues him with the uncanny talents of a vampire. Soon after, he gains the spotlight as the fourth primogenitor, a supernatural being with immense power that many believe to be a myth. Terrified of Kojou's capacity for destruction, the Lion King Organization dispatches a trainee sword-shaman, Yukina Himeragi, to observe him and terminate the boy, considered to be the most dominant vampire in the world, should he pose a threat. Brought together by circumstance, they form an improbable partnership as Kojou grapples with his newfound capabilities while striving to guard the city against various chaotic threats.