Living on the Philadelphia Mainline in the summer of 1875, shy, melancholy Josephine Barrie meets her new neighbor Felicity Archer. They become friends, despite Felicity being a strange little girl dressed in mourning black. Josephine grows fond of her new friend, even as she comes to suspect that Felicity is not what she appears to be. In Philadelphia, the bodies of missing children turn up floating in the Delaware River. Hospital patients die from “acute hemorrhage,” which draws the attention first of Josephine’s physician, and then of Doctor Milo MacFarland, a man who has devoted his life to chasing an elusive vampire.