After losing his parents, eight-year-old Yuuichirou Amane finds solace in the Hyakuya Orphanage, where forming connections proves challenging. When a deadly pandemic wipes out those over 13 years old, vampires emerge from hiding to enslave the remaining humans for their blood. Four years pass, and Yuuichirou, determined to eradicate vampires, clashes with Mikaela Hyakuya, who opts for cunning survival strategies. With Mikaela's aid, Yuuichirou escapes with other orphans, only to face a brutal ambush by vampire officer Ferid Bathory. Yuuichirou flees to the surface and encounters the Moon Demon Company, a vampire-hunting military force. Embracing the chance to seek vengeance and fulfill his goals, Yuuichirou eagerly joins their ranks.