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.)