Unintentionally bestowed with immense power, Kojou Akatsuki transitions from an average high schooler to the legendary Fourth Primogenitor, the most formidable vampire in existence. Struggling to recall the events leading to his transformation, he is monitored by Yukina Himeragi, a young Attack Mage assigned to intervene if his powers spiral out of control. The decision to send a charming girl to oversee a vampire whose strength surges with arousal raises questions, especially on an island teeming with supernatural beings harboring their own agendas involving the Fourth Primogenitor.