In 2012, the world is claimed to have ended at the hands of a man-made virus, devastating the global population and leaving only children untouched. It is during this time that vampires emerge from the depths of the earth, likely following the ancient horrors of darkness believed to be mere legend. Vampires sweep across the world, seizing it with a single violent blow, subjugating the remnants of humanity and taking them to a safe place beneath the surface. The price of this "protection" is to donate blood to those who have captured them. At the age of twelve, Yūichirō and his friend and orphan companion Mikaela planned to escape with the children from Hyakuya Orphanage. However, this results in their deaths, and Mikaela sacrifices himself so that Yūichirō can escape and be rescued by members of the Moon Demon Company, a destruction unit of the Japanese Imperial Demon Army. Four years later, Yūichirō dedicates his life to exterminating vampires and seeking revenge on them for killing his "family."
