Saya Otonashi is a seemingly ordinary girl living a mundane life with her adoptive family in Okinawa City. In fact, her only peculiarities are suffering from anemia and being unable to remember any of her life beyond the previous year. However, Saya's forgotten past quickly comes back to haunt her—one night, she is attacked at school by a creature that feeds on human blood. Just when all hope seems lost, a mysterious man named Haji appears and fends off the creature temporarily. But when her rescuer forces her to drink his blood, Saya suddenly enters a trance and slays the monster with ease, using her own blood as a catalyst. Saya then learns of an organization named Red Shield, founded for the sole purpose of defeating the hellish beasts. Now Saya and Haji must work together with Red Shield to fight these fearsome monsters and unlock the secrets to the girl's past. Throughout the passage of time and the shifts in backgrounds, a particular war continues to leave its mark on history, extending its influence into the modern world. There are two major groups involved in the ensuring war. The first group consists of monsters known as Chiropterans who can change their appearances into that of human beings. They are actually immortals who feed on the blood of the living. The second group is an organisation known as the "Red Shield", formed to track down these monsters and exterminate them. Otonashi Saya is a high-school girl who lives a peaceful life with her family. The only problem is, she has no recollections of her life beyond that of the past year. However, her happy life will be destroyed by an event that will lead her to her destined fate.