When Kojou Akatsuki went to visit his sister in the hospital in middle school, he met a vampire named Avrora. Once he learned she was the key to saving Nagisa's life, he helped Avrora escape. Now, the weapons dealer Zacharias is telling him to hand over Avrora in order to rouse the fourth primogenitor! As Zacharias's plans plunge Itogami Island into peril, the fourth primogenitor finally awakens...