Valancourt and Emily meet, and he laments that her feelings seem to have changed towards him. She says that she still loves him, but asks him to confirm that he is no longer worthy of being her husband, and he agrees. She tells him that this will be their last meeting, and he pleads with her to hope that he will reform. She refuses. Valancourt continues to lament, accusing her of now being in love with Du Pont. Emily agrees to see him one more time, but is not willing to pursue a relationship with him. When they meet the next day, Valancourt is resigned and they tell each other that they will always love one another.
Meanwhile, Montoni's military activities in Italy have attracted the attention of the Venetian government. Morano has also tipped them off that Orsino (who is still wanted as a murderer) is hiding at the castle, which makes them especially eager to capture it. The castle of Udolpho is successfully seized, and Montoni and Orsino are both imprisoned. As a reward for his information, Count Morano is released.
Back in France, Emily is very sad about Valancourt. Eventually, Dorothee reminds her about the story Dorothee promised to tell her. Twenty years ago, the Marchioness came to the Chateau Le Blanc as a young bride, and Dorothee adored her immediately. However, the Marchioness sometimes seemed unhappy and Dorothee gradually learned that the Marchioness had been in love with another man, and was forced to marry the Marquis for his money. Dorothee suspected that she knew who the Marchioness's beloved was, and that the Marquis was sometimes jealous, but also that the Marchioness was always faithful. Dorothee breaks off her story, as she and Emily begin to hear haunting music, and she explains that ever since the death of her mistress, she has been hearing the music sometimes at night, and doesn't know the source. Then, Dorothee explains about the death of the Marchioness: she fell sick with a strange illness that her husband didn't seem to take seriously until it was too late. When she died, her face turned completely black, and the reaction of the doctor led Dorothee to suspect foul play may have been involved. However, nothing was ever openly said. After the death, the Marquis closed up the chateau, went abroad, and died a few years later.
Hearing the story makes Emily more curious about seeing the rooms where the Marchioness lived and died, and Dorothee agrees to take her there. In the rooms, Dorothee explains how everything remains exactly as it was, and shows Emily a portrait of the Marchioness hanging there. The portrait resembles the picture Emily's father was weeping over, and also bears a strong resemblance to Emily herself. Dorothee and Emily become more and more frightened in the room until Emily starts to think she sees figures moving around the room. The two of them flee in terror, and news of the mysterious haunting soon gets out, and frightens all of the servants. The gossip frustrates the Count de Villefort, especially when servants start to quit and refuse to stay in the house. Ludovico offers to spend the night in the rooms in order to prove that there is nothing there, and the Count is pleased with his bravery. The Count and Henri set Ludovico up in the rooms, and he spends the evening reading a book of Provencal tales before falling asleep.
In the meantime, the Baron St. Foix, and his son the young Chevalier St. Foix, have come to stay at the chateau. The Baron is a good friend of the Count, and Blanche and the Chevalier are in love and expected to get engaged soon. In the morning, the Count calls for Ludovico to come out of the rooms, but does not get an answer. After a while, people begin to get alarmed and break into the room, where Ludovico is nowhere to be found. They search the entire castle, but cannot find him, and the disappearance is all the more shocking because there is no way out of the room. With no success finding him, life moves on. Du Pont comes to the castle, and tells Emily that he still loves her, but she tells him she can never return his feelings. The Count urges Emily to consider marrying Du Pont, but she declines and returns to the convent.
Back at the convent of St. Claire, the nuns have heard about the strange disappearance of Ludovico, and talk about how the chateau has been rumored to be haunted. One nun, named Sister Agnes, is particularly agitated, and keeps saying strange things about sin and damnation. Emily also gets word about the fate of Montoni: Orsino was executed for his crime, and Montoni died mysteriously in prison, likely as a result of foul play. With Montoni dead, Emily can now easily claim the estates of her late aunt. Monsieur Quesnel is much more helpful now that his niece is a wealthy heiress, and tells her that she can now come and live at La Vallee. He suggests that Emily travel first to Thoulouse to check in on her new estates there, and then continue on to La Vallee. Emily agrees to leave in a few weeks.
In the interval, the strange happenings at Chateau Le Blanc continue to fuel gossip and panic. The Count and his son also spend the night in the supposedly haunted rooms, and notice nothing unusual. At the convent, Sister Agnes continues to behave strangely, and Sister Frances, one of the other nuns, agrees to tell Emily a bit about her history. Before Agnes became a nun, she was an aristocratic young woman and was in love with a man who was less wealthy than her. Despite her feelings, Agnes' father forced her to marry a different man, and Agnes responded by having an affair. Her husband found out and was going to kill her, so her father helped her escape to the convent and told everyone that she had died. Agnes has been there ever since, living a holy life, but also suspect to strange fits of madness. Emily notes that the time when Agnes entered the convent would be right about the same time when the Marchioness died.
As the time comes for Emily to depart, the Count de Villefort announces that he is tired of the chaotic atmosphere of the chateau. The Baron and Chevalier are going to return to their estate, located in the Pyrenees mountains on route to Gascony, and Blanche and the Count will go with them. The Count suggests they all travel together, but Emily needs to go to Thoulouse first. She proposes that the Baron and Blanche come and visit her at La Vallee after their time at the St. Foix estate.
Valancourt's passivity and shame at his past behavior work against him, generating further miscommunication between him and Emily. Readers will later learn that Valancourt's behavior in Paris was not actually that bad, and if he had explained this to Emily when she confronted him, she almost certainly would have been appeased. However, Valancourt is so ashamed of the way he has acted that he leads Emily to think worse of him than he needs to. As characters, both Emily and Valancourt are almost parodies of idealism and virtue, but they often fail to actually act and resolve challenges. This miscommunication delays the culmination of their relationship even further, and also leaves Emily in limbo.
Clearly, as a single woman, Emily is going to be constantly in peril from unwanted and self-interested suitors. Du Pont seems to be an entirely kind and well-intentioned man who truly loves Emily, but he still poses a risk as a man that Emily would be unhappy with, since her heart belongs to Valancourt. Because he is a good man, there is an even greater risk that Emily might simply give in and agree to marry him. The Count de Villefort is also subtly exerting pressure, and his positive and kind influence is almost more insidious than the cruel manipulations of Montoni. Emily has been lost and adrift ever since the death of her father, and there is a real risk that she will latch on to a new father figure, and trust his judgement, even if he cannot fully understand the complexities of her feelings.
While Emily grapples with the loss of the man she truly loves, and the pressure to marry a man whom she cannot love, more details emerge around two women who suffered similar problems. The Marchioness has been a mysterious presence in the novel ever since Emily first heard her mentioned, but only now does Dorothee reveal the backstory. Not unlike the Signora Laurentini, the Marchioness was torn between her true feelings, and social requirements to marry for more strategic reasons. Unlike the Signora, however, who vanished rather than be trapped in a loveless marriage, the Marchioness trusted her father's judgment and went ahead with marrying the Marquis. Much like Emily's naivety and submissiveness, the Marchioness believed that she should rely on her commitment to loyalty and fidelity. However, Dorothee's story implies that the Marchioness only went on to suffer through an unhappy marriage and a potentially nefarious death. After Emily's experience witnessing the marriage and death of her aunt, this story about the Marchioness has to seem particularly grim: she has to concede that Dorothee's suspicions are plausible, and the Marquis might well have been responsible for the death of his wife.
Sister Agnes' story offers an alternative perspective on the fate of women trapped in a loveless marriage. It will later be revealed that this story is inaccurate, and instead the "cover story" Agnes has used to conceal her true identity (Signora Laurentini). Nonetheless, this story is important because it does reveal some key themes. According to this story, Agnes was also forced into an unhappy marriage, but unlike the dutiful Marchioness, Agnes chose to pursue her passion and have an affair. The two women act as two sides of the same coin, and yet ended up with parallel fates. The Marchioness tried to do everything correctly, and ended up dead at the hands of her husband, while Agnes openly rebelled and acted sinfully, but only narrowly escaped being murdered by her husband. She had to essentially give up any freedom and secular life just to stay alive. Either way, the stories of the two women provide Emily with powerful warnings about what can happen once women come under the sway of a husband.
The fallout from these stories is also leading to the ongoing theme of supernatural happenings within the text. Much like Annette, Dorothee is presented as very gullible and superstitious because she is of a lower-class and less educated. Emily is imaginative and susceptible, but even with these factors, the two women seem to encounter something terrifying in the rooms where the Marchioness died. The terror takes on an almost contagious effect, spreading through the servant population, and irritating the Count de Villefort. The Count's highly rational and masculine approach of skepticism contrasts with the more emotional and imaginative response presented by women such as Emily and Blanche, or servants. Interestingly, it is Ludovico who bravely volunteers to disprove rumors by spending a night in the supposedly haunted rooms. The instance of a servant being the one to take on this heroic task unsettles the expectation of aristocratic men occupying the position of chivalric and romantic hero; Ludovico might choose to step up because he is hopeful of a reward, or the Count might particularly think that the other servants will trust Ludovico's perspective even more. Ludovico's disappearance adds to the many mysteries alluded to in the novel's title, and leaves a lingering sense that there is indeed something dark and suspicious lurking within the Chateau Le Blanc.