Maud Ruthyn is an heiress who lives with her somber, reclusive father Austin Ruthyn in their mansion at Knowl. Through her father and her worldly, cheerful cousin, Lady Monica Knollys, she gradually learns more regarding her uncle, Silas Ruthyn, a black sheep of the family whom she has never met. Once an infamous rake and gambler, he is now apparently a fervently reformed Christian. His reputation has been tainted by the suspicious suicide of a man to whom Silas owed an enormous gambling debt, which took place within a locked, apparently impenetrable room in Silas's mansion at Bartram-Haugh.

Joseph Sheridan Le Fanu was an Irish writer of Gothic tales and mystery novels. He was a leading ghost-story writer of the nineteenth century and was central to the development of the genre in the Victorian era. M. R. James described Le Fanu as "absolutely in the first rank as a writer of ghost stories".