Young and gifted Robert, second son of George Colwan, Laird of Dalcastle, virtually disowned on suspicions of parentage and thus denied the name of his father, is taken in and raised by the fundamentalist leader Reverend Wringham. Despite a propensity towards jealousy, malice, and backstabbing, young Robert soon finds himself initiated into his adopted father's sect, convinced of his predestined salvation and inability to do wrong.

The day of his so-called 'election' to grace, he meets a strange individual that seems to reflect his image and beliefs in every manner, who goes by no other name than Gil-Martin. Together, they spend many a day discussing all facets of doctrine and scripture, stretching the definition of Christianity to its most dangerous extreme, until the young Wringham can no longer resist the charms and flattery of his only friend. With the conviction of the Elect, he gives in to every whim of the coercive fiend no matter how wicked the crime.

Presented as the reconstruction of a local Scottish history rich in character and tradition, The Private Memoirs and Confessions of a Justified Sinner is a landmark text in Gothic literature and a notorious work for its critique of religious fanaticism, that anticipated and defined post-modern existentialist crime fiction and psychological mysteries.