Having lost his parents, eight-year-old Yuuichirou Amane is taken in by the Hyakuya Orphanage, where he struggles to bond with his new family. However, this soon becomes the least of his worries, when a major pandemic suddenly strikes, killing all above the age of 13. Taking advantage of the chaos, vampires—who previously laid dormant underground—rise up and enslave the surviving humans, using them as livestock to harvest blood.