They get two tidbits of information the next morning: first, a servant from the village happens to mention that he saw Rosanna Spearman walking towards the village the day before, wearing a thick veil as though she didn't want to be recognized.
Betteredge figures the servant must be mistaken – since Rosanna spent the whole afternoon in her room, not feeling well, it couldn't have been her.
The second piece of news is that Mr. Candy, the doctor, ended up getting very sick after driving home in his open carriage in the rain after Rachel's birthday party.
This is particularly bad news, since Rachel is so upset about the Moonstone that her mother was thinking of calling for the doctor.
Franklin Blake receives a telegram from his father saying that he's sending the best detective in England, Sergeant Cuff, to help them recover the diamond.
Betteredge meets the detective at the train station.
Sergeant Cuff is lean and grizzled and gray, and looks sort of melancholy.
When they arrive at the house, they go to meet Lady Verinder, who is out in the gardens.
While they wait for her, Sergeant Cuff gets into a debate with the gardener about the right way to grow roses.
Apparently, Sergeant Cuff's favorite hobby is rose gardening.
This seems to be a weird hobby for a famous detective, but Sergeant Cuff is full of surprises.
Cuff has a meeting with Lady Verinder and Superintendent Seegrave, and asks to see Rachel's boudoir afterwards.
Almost the first thing Cuff notices is the smear of paint on the inside of the door.
Seegrave immediately tells him that one of the female servants brushed it with her skirt the day before, but that he didn't know which of them did it. Who cares, anyway?
Cuff cares – he says that there's no such thing as a "trifle" or a mundane detail. Every little thing is important.
He calls Franklin in to ask him whether the paint on the door was wet or dry when the female servants supposedly smudged it.
Well, the servants were in the dressing room at 11am on Thursday morning, and Franklin figures the paint must have been dry by 3am.
So, the paint must have been smeared between midnight, when everyone went to bed, and 3am.
Rachel comes out of her bedroom just then, because she overheard the conversation.
She seems angry that Franklin is helping with the investigation, and acts mean and spiteful towards him.
Franklin is shocked and upset.
Rachel refuses to answer Cuff's questions, and then runs back into her bedroom, slams the door, and starts crying again.
Cuff brushes the incident off, and asks to speak to Penelope.
He also reassures the servants that none of them are under suspicion, because Seegrave had made them all feel paranoid the day before by treating them like suspects.
Now they're all much more cooperative.
Penelope was able to witness to the fact that the paint wasn't smeared at the time that Rachel went to bed after the party – a bit after midnight.
So, clearly, they want to check everyone's clothes to see who has a paint smear – whoever has a smudge of paint on their clothes is probably the thief.
Cuff dismisses Superintendent Seegrave and says that he'll take matters from here.
Seegrave is annoyed, but leaves.
Franklin Blake asks Cuff if he has any ideas yet about who stole the diamond.
Cuff says that he thinks that nobody stole it, but won't explain what he means.