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.