The next day, the baker's man reports that he saw Rosanna Spearman walking toward Frizinghall, though she was supposedly sick in her room. The household also receives word that Mr. Candy has fallen ill riding home in the rain and now has a fever. A telegram arrives from Franklin's father saying that he has sent the well respected Sergeant Cuff in response to Franklin's request for a better investigator.
Sergeant Cuff arrives, and he is soft-spoken and gaunt. Cuff waits in the garden to see Lady Verinder and passes the time by speaking of roses—Cuff's hobby—with the Gardener. Betteredge, at first, has no confidence in Cuff's appearance, but Cuff gradually proves to be perceptive. Cuff examines Rachel's room and notices the smudge in the paint on the door. Asking questions of the helpful Franklin, Cuff determines that the smudge was made overnight during the theft of the diamond, not the next morning by the servants, as Seegrave had thought. During their discussion, Rachel comes out of her room briefly to advise Cuff, "don't allow Mr. Franklin Blake to help you!" Cuff excuses Rachel's rudeness and continues with his investigation.
Cuff tries to improve the disposition of the servants, whom Seegrave has insulted, by asking their help to find the diamond instead of accusing them. Seegrave excuses himself from the case. Cuff urges calm in the household, insisting, "Nobody has stolen the Diamond. The pieces of the puzzle are not all put together yet."
Betteredge informs Lady Verinder that Cuff wishes to see her. She is reluctant, feeling that Cuff brings "misery with him into the house." In Betteredge's presence, Cuff requests another search of the household, this time for a paint- stained dress, not the diamond. Cuff asks to search the wardrobes of everyone, not just the servants, to show goodwill to them. Lady Verinder agrees. Godfrey, who must leave for a Charity meeting in London, gives Cuff the key to his luggage and asks that his luggage be sent to London after it is searched.
Cuff asks for the washing-book as well so he can determine if any garments are missing from the household. Rosanna brings him the book, and Cuff recognizes her as a former thief. Lady Verinder vouches for Rosanna's current good behavior.
Rachel refuses to have her wardrobe searched, and Cuff calls off the entire search, as it would not be thorough.
Cuff and Betteredge take a walk in the garden, and Cuff notices Rosanna lurking in the shrubbery. Betteredge explains Rosanna's love for Franklin and that Franklin often walks in that garden. Cuff accepts this explanation.
Cuff asks Betteredge if anything amiss passed in the servant's hall the night of the theft. Betteredge instinctively protects his fellow servants and says, "no." Cuff senses Betteredge's stubbornness and makes Betteredge feel guilty by telling him that he (Cuff) likes him.
Cuff interviews each of the servants individually—some come out indignant and angry and others proud and happy. After the interviews, Cuff tells Betteredge to let "poor" Rosanna leave the house if she asks to, but to tell him first. Betteredge questions the angry servants and learns that they had reported to Cuff that Rosanna was burning her fire at four in the morning the night after the theft and that Cuff acted as through he didn't believe them. Betteredge realizes that Cuff has cleverly kept the women from putting Rosanna on her guard by pretending to disbelieve them.
In the garden, Betteredge tells Franklin of the servants' report of Rosanna's suspicious behavior. Franklin determines to report Rosanna's guilt to Lady Verinder, but Cuff walks up and stops him from doing so, reminding him that Rachel's guilt is also still at issue. Franklin is insulted at this implication of Rachel and walks away. Cuff asks Betteredge to show him to the Shivering Sand.
In these chapters, the character of Sergeant Cuff is introduced, representing another feature of The Moonstone that would set the standard for future detective novel staple characteristics. Future detective novels would feature the dynamic of the bungling local police (Superintendent Seegrave) replaced by a more intelligent detective (from a nearby metropolis—London in Cuff's case). The particular character of Cuff—his tall, thin figure, his soft- spoken personality, and his idiosyncrasies, such as his love for roses, and whistling "The Last Rose of Summer"—would also set a standard for future quiet, unexpectedly clever, idiosyncratic detectives, including Sherlock Holmes. Cuff is shown within Betteredge's narrative to be superior to Seegrave in detection intelligence but also in his ability to feel compassion. Unlike Seegrave, Cuff exhibits sympathy for Rachel's agitated state, the agitated state of the servants, and Rosanna's love for Franklin. Cuff is perceptive, not just of clues and crimes, but of other characters' positions and feelings. Finally, though he will guess the circumstances of the diamond theft incorrectly, he is still shown to be one of the prophets of the novel when he correctly foreshadows the peculiar non-criminal circumstances of the crime at the end of chapter XII: "Nobody has stolen the Diamond. Wait a little. The pieces of the puzzle are not all put together yet."
The Moonstone revolves around two main mysteries: the current location of the stolen diamond and how the initial crime took place, yet many other smaller mysteries exist within the plot as part of the larger investigation. In these chapters, the smaller mystery that dominates attention is the mystery of why Rachel mistreats everyone, especially Franklin Blake, and is so uncooperative with the investigation. When Cuff begins to suspect Rachel, and to let others in on his suspicion, it creates a division between the characters—specifically Lady Verinder, Franklin Blake, and Betteredge. What began as a united investigative front ends up fractured and distrustful. This internal division and Rachel's uncooperativeness ensure that the crime will not be solved in this early narrative.
Thus far in the novel, Betteredge has simultaneously called attention to, and smoothed over, issues of class in the Verinder household. Betteredge is an older, trusted servant, and thus, while he respects the divisions of the household, he is also allowed to cross them at times (advising Lady Verinder, for example). Yet after the crime is committed, household class divisions become exposed in a fraught way. Thus, the servants are the first ones suspected by Superintendent Seegrave and are submitted to a search. One of Cuff's early triumphs is to level the household, by asking Lady Verinder for permission to search everyone from herself down. This equal treatment successfully mollifies the servants. The other issue in these chapters of exposing class differences in a painful way is Rosanna's love for Franklin Blake. Betteredge is nearly cruel in his dismissal of the folly of that possibility, while Penelope and Cuff are more understanding. Yet even the sympathetic Penelope deems it "monstrous" that Rosanna would ignore class differences in such a way. Issues of class will become less apparent through the rest of the novel, which is not narrated by servants.