Still no one confesses, and eventually the guests head home, but Poirot gestures for Dr. Sheppard to remain behind. Alone, Poirot asks Dr. Sheppard what he thought of the evening, and Sheppard admits he’s baffled. He asks if Poirot knows the murderer’s identity, or if, perhaps, he was trying to pressure the murderer into an outburst. Poirot smiles and confirms that he knows who the murderer is. He promises to walk Sheppard through his process.
Dr. Sheppard continues to think that Poirot trusts him completely, despite the fact that Poirot has gone behind Sheppard’s back time and time again in the interest of learning the truth. It also seems that Sheppard is still confident that he’s committed the perfect crime, and so keeps playing his part of the “Watson” to Poirot’s “Holmes.”
Poirot says he began by considering Dr. Sheppard’s telephone call: if Ralph Paton had really been the murderer, then there would have been no call. He then considers the motive for the call, judging what it accomplished: the murder was discovered that night, instead of the next morning. Perhaps the reason for the call was that the murderer wanted to ensure that he or she was present when the body was discovered.
As Poirot claimed earlier, the phone call is the key to understanding the case. By asking what the phone call accomplished, Poirot developed a sophisticated hypothesis for the murder—one which he hasn’t alluded to or shared with Dr. Sheppard previously.
Poirot next considered the chair pulled out from the wall. The chair blocked the window so that someone standing by the window couldn’t have seen anything lying on the table. Poirot couldn’t be sure what this “thing” was, but he guessed that the murderer wanted to remove it as soon as possible. Perhaps the murderer called in order to come to the house and remove the object. There were four people at the scene of the crime before the police arrived: Dr. Sheppard, Major Blunt, Raymond, and Parker. Parker had nothing to gain by calling—he would have been first on the scene no matter when the body was found.
The chair, as readers may have suspected, turns out to be a crucial part of the case, not a trivial detail. Notice that Poirot has apparently whittled down the suspects to only four people, including Dr. Sheppard. And yet Sheppard doesn’t seem nervous.
Poirot next turned to the object on the table, which, after calling the company, he guessed was a dictaphone. This object would have been very difficult to remove surreptitiously—the murderer would have needed some kind of receptacle. Poirot also deduced that the voice Raymond heard at 9:30 might not have been Roger Ackroyd’s literal voice, but only a recording. This would suggest that Roger was dead at 9:30, and perhaps that the murderer placed some kind of mechanical device to switch the dictaphone on at 9:30, after he or she left. The murderer must also have known Roger well enough to know about the dictaphone.
While discussing the dictaphone in Chapter 23, neither Poirot nor Raymond raised the possibility that the sound coming from Roger’s office at 9:30 was a dictaphone recording, not an actual human being’s voice. However, Poirot has clearly been considering this possibility for some time, confirming that he often keeps his theories hidden from other people.
Poirot next considered the shoeprints. There were three possibilities: 1) They were made by Ralph Paton; 2) They were made by someone else; 3) They were made by someone deliberately trying to incriminate Ralph Paton. Poirot rejected 2) because it was unlikely that someone would have the same kind of shoes as Ralph. Poirot considered 3) and guessed that Ralph had two similar pairs of shoes. One pair was being cleaned on the night of the murder, but the murderer could have stolen the other. This would suggest that Ralph was wearing a third pair of shoes—boots, perhaps. Poirot sent Caroline to determine whether Ralph had boots, obscuring the reason for his inquiry by asking about their color. When Ralph arrived at Poirot’s house that morning, Poirot immediately asked him what he’d been wearing on the night of the murder; Ralph explained that he’d been wearing boots.
The murderer was clearly trying to frame Ralph Paton for the crime by wearing a pair of his shoes; therefore, it follows that the killer would have had access to Ralph’s shoes, and might have known Ralph well enough to visit him at the inn. Poirot, it’s now clear, misled Caroline by asking her to check on the boots’ color (much as he misled Flora by tricking her into thinking that he wanted to test whether it was possible to hear a voice from the terrace). Once again Poirot’s methods are unorthodox but ingenious.
So the murderer, Poirot now tells Dr. Sheppard, must have been at the Three Boars earlier in the day to steal Ralph’s shoes. He also must have had an opportunity to steal the dagger after Flora Ackroyd examined the silver table. The murderer must have been mechanically minded, must have known Roger Ackroyd well enough to know about his dictaphone, must have been carrying a receptacle to carry the dictaphone—such as a black bag—and must have been alone in the study shortly after the crime was discovered. This person must, in fact, be Dr. Sheppard.
Here, we come to the novel’s surprise ending: Dr. Sheppard is the killer. While this might not seem that surprising for contemporary readers, it was pretty shocking in the 1920s. At the time, mystery novels adhered to a very rigorous format, such that the narrator of the book was automatically considered a trustworthy character (for example, in the Sherlock Holmes novels, the trustworthy Dr. Watson narrates). Nevertheless, Christie twisted the conventions of the mystery novel by making the narrator the killer, winning both criticism and praise in the process. (This is an example of a “twist” that’s partly ruined by how often it’s been copied by other works since its publication. It’s fairly common nowadays—see Gillian Flynn’s Gone Girl for a good example.)