Cuff enlists Betteredge's help with the investigation and asks him to stop protecting Rosanna, as Cuff has no intention of prosecuting her. Cuff believes that Rosanna is acting as "an instrument in the hands of another person." Cuff believes that Rosanna realized that one of her gowns is stained with paint and went to Frizinghall to gather the materials to make another, which she did that night, hence the fire in the early morning hours. Cuff has followed Rosanna today to a house in the fishing village of Cobb's Hole. Cuff proposes to track the rest of her movements by her footprints in the sand. The footprints reveal that Rosanna walked into the water, perhaps to hide something.
Betteredge, having caught the "detective fever," agrees to take Cuff to Cobb's Hole to inquire at the house Rosanna visits—the house of a fisherman named Yolland, Yolland's wife, their son, and their daughter. Rosanna and the daughter, Limping Lucy, are both deformed and had become friends.
At the Yollands', Cuff drinks gin with Mrs. Yolland and gains information about Rosanna from her by being solicitous and sympathetic. Rosanna has come to the Yollands' that day and told them that she is leaving her job at the Verinder house. She used Lucy's room to write a long letter that didn't need postage. She bought a waterproof "old japanned tin case" and two dog chains from Mrs. Yolland.
Cuff assumes that Rosanna has hidden something in the tin case and sunk it in the water or quicksand, secured by the chains. He believes she has hid the stained dress or nightgown but doesn't know why she wouldn't just destroy it. Back at the Verinder house, Cuff learns that Rosanna has been back an hour and bets Betteredge that Rachel has decided to leave the house within the last hour. His bet turns out to be correct, though Betteredge refuses to believe that Rosanna's arrival and Rachel's decision to leave are connected.
Lady Verinder calls Cuff and Betteredge into her room and informs them that Rachel wants to stay with her aunt, Godfrey's mother, in Frizinghall. Cuff asks the Lady to delay Rachel's departure until two o'clock the next day, while he goes to Frizinghall in the morning.
Betteredge finally concedes to Cuff that there is something wrong with Rachel. Cuff believes that Rachel has stolen her own diamond. Betteredge begins to hate Cuff for this suspicion.
Later in the evening, Betteredge reminds Cuff that the Indians will be released from jail tomorrow. Cuff resolves to question them in the morning, with Murthwaite's help.
Betteredge runs into Rosanna, looking pained, in the hall. She is running from Franklin, who reports that Rosanna had come into the billiard room wishing to speak to him but had left in a frenzy because he would not look at her. Franklin meant no offense to her but believes she was about to confess the theft to him. Betteredge goes to check on Rosanna's hurt feelings, but she declines visitors.
Betteredge finds Cuff sleeping on some chairs outside Rachel's room. Cuff is prepared to discover any overnight private communication between Rosanna and Rachel.
Cuff discovers no communication between Rosanna and Rachel overnight. In the morning, Cuff approaches Franklin and Betteredge walking outside. Franklin is still cold to Cuff because of Cuff's suspicions of Rachel. Cuff asks Franklin what Rosanna said to him the night before, but before Franklin can answer, Rosanna herself appears ahead of them with Penelope behind her. Cuff notices her and begins speaking louder about the conversation between Rosanna and Franklin. Franklin, pretending not to notice Rosanna, reassures Cuff, "I take no interest whatever in Rosanna Spearman." Rosanna overhears and lets Penelope take her back in the house. Franklin regrets his harsh words but explains to Betteredge that he used them to protect both himself and Rosanna from Cuff's inquiries. Franklin asks Betteredge to make it up to Rosanna.
Cuff goes to Frizinghall. Penelope asks Betteredge to visit Rosanna and cheer her. Betteredge tries to speak with Rosanna, but she acts as though entranced. Betteredge urges her to "make a clean breast of it!" but Rosanna insists she will speak only to Franklin and returns to her work. Betteredge resolves to speak with Lady Verinder of Rosanna's troubled state, but the Lady is shut up in a room with Rachel. Betteredge hears Cuff return from Frizinghall.
In Chapter XV, Cuff's ability to sympathize with others becomes a detecting tactic, as he solicits information about Rosanna from the Yollands by pretending to be sympathetic and manipulating Mrs. Yolland's response. The information he gets from Mrs. Yolland—information about Rosanna's purchases of a tin case and chains—is information about object clues. Object clues now make up the basis of any detection story. Rather than the suspicious behavior of individuals, objects usually provide the answers to the mystery. This tactic of organization owes much to publicized court cases, in which objects are entered as incriminating evidence.
The theme of intoxication or drugged states recurs strongly in these chapters and is used to describe the state of the Verinder household after the crime has been committed. Betteredge admits to having caught "detective fever," under which influence his actions and interests are not his own. Further on, he explains, "The horrid mystery hanging over us in this house gets into my head like liquor, and makes me wild." Thus, drugged states of mind are connected with both the mystery of the exotic Moonstone and with the obsessive behavior necessary to detect the thief. The theme is eventually related, also, to the particularly desperate lovesickness of Rosanna Spearman. When Betteredge confronts Rosanna about her suspicious behavior toward Franklin Blake and tries to comfort her about Franklin's rejection of her, Rosanna is continually described as acting "like a woman in a dream." Just as drugged or intoxicated individuals have little power over their thoughts or actions, Rosanna is described as being "like a creature moved by machinery."
It is in part because of this helpless behavior that Rosanna is consistently treated as an object of pity by everyone, including Sergeant Cuff. Though Rosanna clearly has something to hide in the wake of the crime, no one except Franklin Blake, who is in search of a scapegoat to clear Rachel from suspicion, believes that she could have committed the crime without orders from another.
The surveillance aspect of the detection process emerges strongly in these chapters. Cuff practices his own surveillance campaign—he eavesdrops on Franklin and Rosanna's conversation, he follows Rosanna's footprints on the Shivering Sand, and he hides overnight in the hallway in hopes of spying on a clandestine meeting between Rosanna and Rachel. Yet other characters practice surveillance as well, such as Penelope's spying on Rosanna's love of Franklin. A big country house such as the Verinder house provides ample opportunity for this type of observation and features of the house seem to figure into the atmosphere of surveillance. We see Betteredge and Cuff standing outside looking at Rachel's upstairs window, and short hallways nearly hide the identity of eavesdroppers.
In these chapters, Rosanna remains the main object of investigation, though she is not primarily under suspicion. Cuff and Betteredge track her movements, which lead them outside the walls of the Verinder house. Rosanna visits both Frizinghall and Cobb's Hole, thus the setting of the novel shifts from the Verinder house for the first time with Cuff and Betteredge's visit to Cobb's Hole. Though the movements of characters include Frizinghall, the town itself will not be shown at all in this narrative, as Betteredge, the narrator, must remain close to the house in his capacity as steward.