A young man, convinced of his divine election, descends into a series of calculated murders under the influence of a mysterious, doppelgänger-like stranger. Robert Wringhim, a religious zealot, believes his status as a 'justified sinner' grants him immunity from moral consequence. He is guided by a sinister figure named Gil-Martin, who encourages his violent impulses against those deemed enemies of his faith. The narrative framework is complex, utilizing a dual-perspective structure that contrasts an objective editorial account with the subjective, increasingly erratic memoirs of the protagonist himself. This structure forces the reader to navigate the tension between theological fanaticism and psychological fragmentation.

Readers and critics frequently highlight the novel's ahead-of-its-time approach to psychological complexity and the unreliable narrator. Discussion often centers on the ambiguity of the antagonist, questioning whether Gil-Martin represents a literal demonic entity or a manifestation of the protagonist's fractured psyche. The pacing is noted for its shift from a detached historical account to a claustrophobic, intense personal descent into madness. Scholars often emphasize the work's biting satire of religious hypocrisy and its significant influence on the development of the Gothic genre. The text remains a subject of intense academic interest due to its layered, multi-vocal construction and its enduring relevance to themes of moral absolutism.