Acting as English liaison to the Imperial Fifth Regiment of Hanoverian Foot, Major Lord John Grey finds himself in the town of Gundwitz during the Seven Years' War, and drawn into investigating a series of mysterious deaths among the encamped German and English soldiers. The local rumor is that a succubus is to blame, but Grey chooses logic and reason over superstition, and pursues a line of inquiry into the deaths of an English private and a Hanoverian soldier, as well as the involvement of the nearby gypsy population and a local saint by the name of Orgevald. Grey must also contend with his increasingly ambiguous relationship with Stephan von Namtzen, as well as the unwanted attentions of a Prussian princess.