Kojo Akatsuki is an ordinary high-school boy--but when that high school is in the Demon District of Itogami City, "ordinary" means something rather different. He might be a vampire, for example. He might even be the most powerful vampire of all time. Middle schooler Yukina Himeragi, meanwhile, is the attack mage sent to observe him--a cute girl sent to observe a massively powerful vampire boy whose abilities activate whenever he gets aroused, on an island city full of all manner of supernatural individuals, each with their own shadowy agenda... What could possibly go wrong?