One day, a mysterious virus appeared on Earth which killed every infected human over the age of 13. At the same time, vampires emerged from the world's dark recesses and enslaved mankind. After escaping one of the Vampire Concentration Camps, Yuuichirou Hyakuya joins the Vampire Assassination Elite Unit(VAEU), a mercenary team where specialized soldiers are assigned to kill vampires who are becoming a potential threat to the Moon Demon Army. After being promoted to Corporal Sergeant, Yuuichirou is given a special assignment-to assassinate Mikaela Hyakuya, an old childhood friend in which he doesn't remember.