In Chapter 26, it becomes abundantly clear that Dr. Sheppard is the killer and has been an unreliable narrator. Poirot reveals that not only can Sheppard not be trusted:
I went through similar actions [to those you took when you murdered Ackroyd] the other day, when you were with Mrs. Ackroyd—it took ten minutes exactly.
The question of when Ackroyd was killed and how long it would have taken various suspects to get to the right places at the right times has been important throughout the entire case. Poirot reveals that, unbeknownst to Sheppard, he has investigated the doctor and determined that his timeline supports the hypothesis that he is the killer.
This revelation is a reversal of the dramatic irony Sheppard has enjoyed throughout the book. Sheppard has inserted himself into the center of the murder case, hiding his guilt in plain sight by working with Poirot and trying to demonstrate who else had the means and motive to kill Ackroyd. He has been careful to craft a narrative in which almost all of his lying occurs by omission, so that he always knows just a bit more than everyone else, including Poirot and the reader. Here, Poirot reveals that even the "mastermind" Sheppard never knew the whole story. It was a mistake for Sheppard to believe that he always had the upper hand.
Poirot delivers a lesson here not only to Sheppard, but also to the reader. Even more than the villagers, the novel's readers have had every reason to trust Dr. Sheppard to tell the truth because one of the "rules" of detective fiction at this time was that the narrator is never the murderer. But just like Poirot must assume that no one is ever telling him the whole truth, the reader finds out here that even a narrator can have reasons to lie or conceal bits of the truth. This novel has been lauded as one of the best "whodunnit" mysteries because Christie subverts the expectation that anyone is wholly trustworthy. Poirot, who again and again insists that everyone has something to hide, models a kind of suspicious approach to reading a mystery novel that has been popular ever since Christie introduced it. Instead of trusting the narrator, Christie suggests, readers should become amateur detectives in their own right and piece together the mystery for themselves.
