On an American military base in Japan, a new kind of vampire emerges: Teropterids. They are monstrous shape-shifting creatures that can only be killed with special swords. A mysterious girl named Saya is the last 'original', the only person capable of dealing with the menace of these monsters. Posing as a student at the base's school, Saya races to hunt down the beasts before they turn an ordinary Halloween bash into a bloody massacre.