Set in early eighteenth-century Scotland, the novel recounts the corruption of a boy of strict Calvinist parentage by a mysterious stranger under whose influence he commits a series of murders. The stranger assures the boy that no sin can affect the salvation of an elect person. The reader, while recognizing the stranger as Satan, is prevented by the subtlety of the novel's structure from finally deciding whether, for all his vividness and wit, he is more than a figment of the boy's imagination.