A young man, convinced of his status as one of the elect, commits a series of murders under the influence of a mysterious, doppelgänger-like figure. Robert Wringhim, raised by a fanatical Calvinist minister, adopts a distorted theological belief that his salvation is guaranteed regardless of his actions. This conviction leads him to commit heinous crimes, guided by a stranger who mirrors his own appearance and temperament. The narrative employs a dual-structure, alternating between the editor's objective account and the protagonist's own subjective, often unreliable, memoirs. The world is defined by the rigid, oppressive religious climate of 18th-century Scotland, where the boundaries between divine predestination and psychological instability blur.

Discussion often centers on the novel's sophisticated use of narrative ambiguity, which leaves the reader questioning whether the protagonist is truly possessed by a demonic entity or suffering from a severe psychotic break. Readers frequently highlight the chilling effectiveness of the doppelgänger trope, noting how it serves as a physical manifestation of the protagonist's fractured psyche. Critics often point to the text's biting satire of religious hypocrisy, which remains sharp despite the passage of time. The pacing is frequently described as deliberate, allowing the psychological tension to build alongside the increasingly erratic behavior of the narrator. Many readers find the contrast between the editor's rational tone and the protagonist's fevered prose to be a defining feature of the reading experience.