The novel is a first-person narrative told from the point of view of the adolescent girl Maud Ruthyn, an heiress living with her sombre, 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.