The 1920s was a decade marked by significant change and turmoil, as monstrous demons emerged across America. To counter this threat, the holy organization known as the Order of Magdalene was founded. The organization's New York branch houses the young and reckless Sister Rosette Christopher, along with her partner Chrno. Charged with eliminating demonic threats, this well-known team performs their duties effectively, even though their missions often result in extensive collateral damage. Nevertheless, both Rosette and Chrno are motivated by their dark pasts. By exterminating demons, Rosette hopes to locate her missing brother Joshua, who was taken by the sinner and demon Aion, a figure with whom Chrno also shares a bloody history. The pair must battle the growingly dangerous demonic threat and uncover its origin, all while continuing their search for the truth regarding Joshua's disappearance.