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 Gothic castle, 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. For details she relied on travel books, which led her to make several anachronisms. 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. He and others frustrate Emily's romance with the dashing Valancourt. Emily also investigates a relationship between her father and the Marchioness de Villeroi, and its connection to Castle Udolpho.
Emily St. Aubert is the only child of a landed rural family whose fortunes are in decline. Emily and her father share a notably close bond in a shared appreciation for nature. They grow still closer after her mother's death from illness. She accompanies him on a journey from their native Gascony, through the Pyrenees to the Mediterranean coast of Roussillon, over many mountainous landscapes. During the journey, they encounter Valancourt, a handsome man who also feels an almost mystical kinship with the natural world. Emily and Valancourt fall in love.
Emily's father succumbs to a long illness. Emily, now orphaned, is forced by his wishes to live with her aunt, Madame Cheron, who shares none of Emily's interests and shows little affection for her. Her aunt marries Montoni, a dubious nobleman from Italy. He wants his friend Count Morano to become Emily's husband and tries to force him upon her. After discovering that Morano is nearly ruined, Montoni brings Emily and her aunt to his remote castle of Udolpho.
Emily fears she has lost Valancourt forever. Morano searches for Emily and tries to carry her off secretly from Udolpho, but Emily's heart still belongs to Valancourt, and she refuses. Morano's attempted escape is discovered by Montoni, who wounds the Count and chases him away. In subsequent months, Montoni threatens his wife with violence, trying to force her to sign over her properties in Toulouse that will otherwise go to Emily on his wife's death. Without resigning her estate, Madame Cheron dies of a severe illness caused by her husband's harshness.
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. Returning to her aunt's estate, Emily learns that Valancourt has gone to Paris and lost his wealth. Nonetheless, she takes control of the property and is reunited with Valancourt in the end.