Dr. Sheppard has just brought Flora Ackroyd to her room. Coming downstairs, he speaks to the inspector. The inspector asks for a better description of the stranger, but Sheppard can’t give him one—it was dark. He has a feeling that the man was trying to disguise his voice.
Sheppard’s testimony further suggests that the stranger was someone whom Sheppard has already met (given that he seemed familiar, and was trying to disguise his voice).
The inspector, whose name is Davis, brings Dr. Sheppard to the study to ask him more questions. He asks Sheppard about blackmail—a possibility that Parker has just raised. Sheppard realizes that Parker must have been listening at the door, and Davis explains that he’s suspicious of Parker. Sheppard decides to tell Davis the truth: he explains what Roger told him, including the information about Mrs. Ferrars and the blackmailer. Davis realizes that Roger’s murderer might have been Mrs. Ferrars’s blackmailer. Sheppard suggests that the blackmailer may have been Parker himself, and Davis agrees.
Inspector Davis initially suspects that the butler committed the crime. By the early 20th century, the revelation that “the butler did it” had become so common in detective stories that the most popular mystery writers made a point of never using it. (In fact, one of “Knox’s Ten Commandments of Detective Stories,” a set of rules that many mystery writers used, is that the butler should never be the killer. So it’s probably meant to be a sign of Davis’s cluelessness that he assumes that Parker is the criminal—any reader of mystery novels would know this can’t be right.
Inspector Davis next turns his attention to the murder weapon: a beautiful, ornate blade. Sheppard examines it and notes that the murder was clearly committed by a right-handed man; furthermore, Ackroyd may have died without knowing who killed him. Davis notices fingerprints on the weapon. He shows it to Geoffrey Raymond, who recognizes it as a gift from Tunis, which Major Blunt gave Ackroyd—Major Blunt promptly confirms this. The knife, Raymond explains, was kept in the silver table. This prompts Sheppard to recall that he heard the sound of the table lid being shut. He can’t, however, remember if the knife was there when he examined the table. Davis summons Miss Russell, who recalls seeing the silver table open and shutting it. She can’t remember if the knife was there.
Here, readers learn why the sound of the silver table shutting was so important: it housed the Tunisian dagger with which Roger Ackroyd was murdered. This is an important step forward for the investigation, because the inspector can determine when the dagger was still in the table—and therefore, who would have been able to steal it. However, neither Sheppard nor Russell can recall whether or not the dagger was present.
Davis thanks the men for their help and says he’ll be back. Before he leaves, however, he asks Parker’s “opinion of a small pocket diary.” Raymond, realizing what this means, tells Sheppard that Parker is clearly a suspect. He suggests that they donate their fingerprints to Davis. Together, the men present Davis with cards, which they’ve marked accordingly.
Davis obtains Parker’s fingerprints by getting him to handle the diary. The fact that Raymond and Sheppard then offer their fingerprints willingly would suggest that they’re not concerned about their fingerprints being on the dagger.
Dr. Sheppard returns to his home, where Caroline is waiting. He doesn’t mention the blackmailing, but tells Caroline everything else. Caroline finds it absurd that Parker could be the murderer.
Caroline really does seem to be modeled on the ideal mystery novel reader: like any seasoned Agatha Christie fan, she knows the butler couldn’t have done it—it’s just too much of a cliché.