Features considerable changes regarding Agatha Christie's novel, like the fabrication of new characters (Lord Boynton, Nanny Taylor, and Sister Agnieszka), the omission of others (such as Nadine Boynton and Amabel Pierce), the retooling of existing characters and even the reasons of the killer.
Mark Gatiss (Leonard Boynton) also worked behind the scenes on Poirot; writing the screenplays for Cat Among the Pigeons (2008), Hallowe'en Party (2010), and The Big Four (2013).
Dame Celia tells Poirot she is reading The Perfumed Garden to Lord Boynton to cheer him up. An odd choice, since The Perfumed Garden is basically a sex manual.
In the Christie novel, there is no Lord Boynton. Mrs. Boynton (not a Lady as her late husband was not a baron or peer) was a widow. And the character's name was Lady Westholme not Dame Celia Westholme.
A 1945 West End theatrical adaptation of the original 1938 novel eliminated Poirot's character and changed the identity of the killer. It is notable that Joan Hickson, later to play Christie's Miss Marple, was in the cast.