A religious zealot, convinced of his own divine election, descends into a murderous spree under the influence of a mysterious, malevolent companion. Robert Wringhim, the protagonist, operates under the dangerous theological assumption that his status as a 'justified' soul grants him immunity from moral law. He is guided by a sinister stranger who mirrors his own darkest impulses, leading to a series of escalating crimes. The narrative is presented through a complex, dual-perspective framework that contrasts the objective account of an editor with the subjective, often unreliable memoirs of the sinner himself.

Discussion often centers on the innovative narrative structure, which forces readers to reconcile two conflicting accounts of the same events. Readers frequently highlight the chilling effectiveness of the doppelgänger, noting how the stranger functions as both a physical entity and a manifestation of the protagonist's fractured psyche. Critics often point to the book's early use of psychological realism, which predates many later developments in the genre. The balance between theological satire and genuine horror remains a primary subject of academic analysis. Many readers find the ambiguity of the ending to be the most compelling aspect of the work, as it refuses to provide a simple resolution to the moral questions posed.