On Itogami Island's Demon District, Kojou Akatsuki leads a standard high school existence until a sudden event grants him the colossal power of a vampire. This change draws immediate attention, exposing him as the Fourth Primogenitor, a legendary and terrifying vampire previously believed to be mere myth. Fearing the destruction he might unleash, the Lion King Organization sends Yukina Himeragi, an apprentice sword-shaman, to watch over Kojou and eliminate him should he turn dangerous. As they navigate their hesitant partnership, Kojou fights to control his new abilities while both strive to shield the city from rising chaos and threats.