Her fourth and most popular novel, The Mysteries of Udolpho tells of Emily St. Aubert, who suffers misadventures that include the death of her mother and father, supernatural terrors in a gloomy castle, and machinations of Italian brigand Signor Montoni.

[2][3] The Mysteries of Udolpho is a quintessential Gothic romance, replete with incidents of physical and psychological terror: remote crumbling castles, seemingly supernatural events, a brooding, scheming villain and a persecuted heroine.

Modern editors note that only about a third of the novel is set in the eponymous Italian Gothic castle,[4] while tone and style vary markedly between sections of the work, to which Radcliffe added extended descriptions of exotic landscapes in the Pyrenees and Apennines, and of Venice, none of which she had visited.

The novel, set in 1584 in Southern France and Northern Italy, explores the plight of Emily St. Aubert, a young French woman orphaned by the death of her father.

She is imprisoned in Castle Udolpho by Signor Montoni, an Italian brigand who has married her aunt and guardian Madame Cheron.

Many frightening but coincidental events happen in the castle, but Emily manages to flee with the help of a secret admirer, Du Pont, also a prisoner there, and of the servants Annette and Ludovico.

[8] Modern critics have noted the influence of The Mysteries of Udolpho on the works of many later writers, including Edgar Allan Poe,[9] John Keats,[10] and Henry James.