In 2012, a man-made virus devastates the world, sparing only children from its deadly grasp. Amidst this chaos, vampires emerge from hiding, possibly accompanied by ancient terrors once thought to be mere legends. These vampires swiftly conquer the earth, subjugating the surviving humans and leading them underground under the guise of protection, which comes at the cost of providing blood to their new overlords. At twelve years old, Yūichirō and his fellow orphan Mikaela, along with other children from the Hyakuya Orphanage, attempt to flee. Their escape plan ends in tragedy, with Mikaela sacrificing himself so that Yūichirō can survive, ultimately rescued by the Moon Demon Company, a vampire-hunting unit of the Japanese Imperial Demon Army. Four years later, Yūichirō is driven by a desire for vengeance, dedicating his life to eradicating vampires for the slaughter of his “family.”