Set in the artificial island metropolis of Itogami City, a sanctuary for magical beings and demons, the story centers on Kojo Akatsuki, an ordinary high school boy whose life shatters overnight. Following a mysterious incident, Kojo unknowingly inherits the immense powers of the Fourth Primogenitor—the legendary, destructive vampire king feared by nations worldwide. To monitor this catastrophic threat and neutralize him if he loses control,
the Lion King Organization dispatches Yukina Himeragi, a novice sword-shaman. Armed with a powerful anti-primogenitor spear, Yukina is ordered to shadow Kojo constantly, inadvertently enrolling in his school and moving in near him.Despite initial tensions, Kojo and Yukina form an
uneasy alliance. Kojo desperately wants to retain his humanity and live a normal life, but his dormant bloodlust constantly threatens to awaken whenever he faces severe trauma or magical combat. To unlock his full, terrifying arsenal of familiars—monstrous avatars capable of cataclysmic destruction—Kojo must consume blood. This biological necessity forces him into deep emotional and physical intimacy with Yukina, who reluctantly becomes his primary blood-donor. Together, they navigate the dangerous political underbelly of Itogami City, which is constantly targeted by rogue terrorists, corrupt corporations, ancient alchemists, and rival supernatural factions seeking to harness or destroy the Fourth Primogenitor.As the series progresses, the threats escalate from localized terrorist cells to global conspiracies. Kojo and Yukina are joined by a colorful cast of allies, including Aiba Asagi, a genius teenage hacker harboring deep secrets, and Natsuki Minamiya, Kojo’s deceptively youthful homeroom teacher who is actually a powerful spell-weaving warden. They face formidable foes like the artificial lifeform Kanon Kanase and the ruthless mastermind Avrora Florestina, whose tragic history is intricately tied to the origin of Kojo’s vampiric curse. Through numerous grueling battles, Kojo learns to accept his monstrous destiny rather than fear it, mastering his devastating familiars one by one to protect the people he loves.The narrative reaches its peak when the barrier protecting Itogami City begins to collapse, drawing the attention of other Primogenitors and the World Guard, who plot to eradicate the island and its supernatural inhabitants. Kojo is forced to embrace his true identity as the king of vampires, risking his own sanity and mortality. In a final, explosive confrontation, Kojo unites his friends, harnesses the full extent of his blood-fed power, and defeats the orchestrators of the island's destruction. By the story's climax, Kojo successfully averts global annihilation, cementing his place not as a harbinger of doom, but as the unlikely protector of Itogami City, with Yukina faithfully by his side as both his partner and shield.