St. Aubert lives at La Vallée in a peaceful part of Gascony (in France) with his wife, Madame St. Aubert, and adult daughter, Emily. St. Aubert does his best to give his daughter a well-rounded education and an appreciation of nature, and she eagerly takes to these lessons. Indeed, she loves nature so much that she often composes poems about it. Madame St. Aubert’s relatively sudden death, however, breaks up this idyllic world, and St. Aubert’s own poor health forces him to travel with Emily to Languedoc.
On the way to Languedoc, Emily and St. Aubert meet Valancourt, a dashing and generous young man who very quickly takes a liking to Emily and finds excuses to keep traveling with her. Unfortunately, during the journey, St. Aubert dies. Before he passes away, he instructs Emily to go find some of his old papers and burn them without looking at their contents. Emily burns the papers, catching just a glimpse of what they say that both intrigues and terrifies her. She keeps a small miniature she finds, which portrays a mysterious young woman and which she once saw her father secretly kiss when he thought he was alone.
As Emily’s only surviving relative, Madame Cheron becomes her new guardian. When Valancourt shows up to court Emily, Madame Cheron quickly discourages the match because she believes that Valancourt’s family isn’t wealthy enough. She even accuses Emily of acting improper with Valancourt behind her back. Meanwhile, because Madame Cheron is a widow, she is trying to get remarry. She ultimately marries Montoni, an Italian who appears to be a wealthy gentleman.
Emily must go with Montoni and her aunt (now called Madame Montoni) to Venice, where she longs for Valancourt. But Montoni tricks her into signing a letter saying she consents to marry Count Morano, Montoni’s choice for her. However, Montoni’s debts in Venice, combined with his obligation to help his associate Orsini (who is wanted by the Senate for arranging an assassination), cause Montoni to flee the city for a castle at Udolpho that he inherited but hasn’t used for a long time. He takes Emily and Madame Montoni with him.
The castle of Udolpho is a mysterious place, where Emily often remains in her chamber for her own safety. Her main companion there is her faithful servant, Annette. From her chamber, Emily can hear strange music at night, and one of the castle’s biggest mysteries is a black veil, which contains something so horrifying that Emily can’t even comprehend it when she lifts the veil to look at what’s underneath. It causes Emily to faint.
One night, Count Morano invades the castle and tries to abduct Emily by coming in through a secret passage to her room. Montoni, who has turned against Count Morano by this point, duels him and wins, ensuring that Emily remains trapped in the castle.
Emily doesn’t get to see her aunt, Madame Montoni, for a long period of time, and she begins to fear that perhaps her aunt is dead. At last, Emily gets to see her aunt, who is alive but on the verge of death. Montoni wants his dying wife to sign over all her estates to him, but Madame Montoni refuses and soon dies, leaving everything to Emily and causing Montoni to pressure Emily so he can take her estates and money. All the while, Annette begins a romance with the brave servant Ludovico.
Eventually, bandits begin attacking the castle, partly in retaliation for raids that Montoni led with his band of mercenaries. During the confusion during one nighttime raid, Emily, Annette, Ludovico, and some others manage to make an escape. Montoni and Orsino later get captured, and they both die.
Emily returns to her aunt’s estate (which she now owns) and also spends time visiting the family of the Count De Villefort. She is eager to see Valancourt again, but the Count informs her that living in Paris has corrupted Valancourt: Valancourt gambled all his money away and even spend time in prison. Emily is horrified by this, and when she finally reunites with Valancourt, she decides that the two of them must part ways forever.
While still feeling betrayed by Valancourt, Emily begins to unravel some of the mysteries related to the Marchioness De Villeroi. She died at a young age and lived at the Count De Villefort’s house before he inherited the property. It turns out the Marchioness was murdered by the Marquis De Villeroi and Signora Laurentini, who were secretly lovers and who wanted to get rid of the Marquis’s wife to remove any obstacles to their love. But after carrying out the deed, the Marquis regretted his actions and decided to move away and spend the rest of his life alone. Meanwhile, Signora Laurentini changed her name to Agnes and joined a convent, where she remained haunted by her deeds right to the moment of her death. It is during a deathbed conversation with Agnes/Laurentini herself that Emily learns most of this story.
Emily wonders how St. Aubert relates to this story, as she learns that the Marchioness is the woman in his secret miniature. She suspects at first that her father may have been the Marchioness’s lover, but she ultimately learns the truth: The Marchioness was his sister.
Upset by Emily’s rejection, Valancourt tries to prove his worth, generously giving much of his money to support Emily’s old servant Theresa, whom she often worries about. Emily eventually learns that Valancourt’s actions in Paris were not as bad as she feared and that particularly after going to prison, he reformed. This clears the way for Emily and Valancourt to finally get married. They have a large wedding on the same day as Count Villefort’s daughter, and they live together happily and relatively modestly in La Vallée.
