This episode was based on the novel Cards on the Table, first published in 1936. It was adapted for television by Nick Dear and directed by Sarah Harding.
Script versus novel
It's quite surprising, actually, that the writer who delivered perhaps the most faithful script of Series Nine (The Hollow) should come back for Series Ten and deliver one of the more controversial scripts. I happen to like it (for the most part), but he does make some rather peculiar changes that I can't really say I see any reason for making. I enjoy Dear's later adaptations, and they are generally quite faithful, so I wonder what got into him while writing this. In any case, let me list the changes.
First, some new characters are added. Colonel Race is replaced by Colonel Hughes, and Superintendent Battle is replaced by Superintendent Wheeler. The first change can be explained quite easily. James Fox, who portrayed Colonel Race in Death on the Nile was unavailable, and the role would have to be re-cast. To avoid this, Dear evidently added an original character. A perfectly acceptable decision, and I think Robert Pugh does a good job with the part, too.
The second character substitute is somewhat more difficult to explain. Battle had never been portrayed before in the series, so there is no need to remove him for those kinds of reasons. It seems the reason he was removed was because Dear (or the producers?) wanted to increase the number of suspects by implicating one of the 'sleuths', and Battle/Wheeler was the obvious choice. Throughout the investigation, the Wheeler character shows signs of having a personal interest in the case (he knows that Shaitana was Syrian, he is keen to accuse Dr. Roberts of the crime, and it is revealed that he staged a break-in to search Shaitana's house for some compromising photographs. Poirot discovers the compromising photographs at a somewhat suspicious-looking photographic studio and confronts Wheeler with them after the denouement. Whether Wheeler is a closeted gay man or the photos show something else entirely, we shall never know. In any case, this is a peculiar addition.
Third, Dear decides to make Dr. Roberts gay. He has an affair with Mr. Craddock, rather than Mrs. Craddock, and it's Mrs. Craddock, not Mr. Craddock, who threatens to report him. I can't really say I understand that change either. The only reason I can think of is that Dear felt the 'threat' of an affair with a female patient wouldn't be enough of a motive for Roberts to commit murder. To have his homosexuality revealed in a society that had deemed homosexual acts illegal, however, would certainly be considered a reason for wanting to silence those who know his secret. I don't say that this explains why it was necessary to change his motive, but I do think it makes it more understandable - and almost acceptable. Fourth, Dear removes the second murder (and the faked suicide letters), but that entire plot was almost unbelievable (how was Dr. Roberts supposed to be aware that she was deadly ill and that she was thinking of taking the blame - not to mention how he could get hold of her handwriting).
Fourth, the third murder (SPOILER) is changed so that Rhoda (not Anne) drowns, and Rhoda (not Anne) attempts to kill her. Also, Despard rescues Anne and not Rhoda. In fact, Rhoda is supposed to be a possessive friend (in Poirot's words to Anne Meredith: 'you were her slave'). Again, I find it difficult to understand why there was a need to change this from the novel (but, of course, it allows for mother and daughter to be reconciled rather than murdered). Finally, as I've already implied, Anne Meredith is Mrs Lorrimer's daughter here. This allows for a much more believeable and emotional 'confession' scene.
Apart from the above mentioned changes, however, Dear's adaptation stays fairly close to the source material (indeed, it's only in the final thirty minutes or so that things go off in... unexpected directions. All in all, however, I'm inclined to regard this as a perfectly acceptable adaptation, with some flashes of perfection (the bridge scenes, the dynamic between the four sleuths etc are all superbly done).
Directing, production design, locations, soundtrack
Sarah Harding's direction is wonderful. I particularly like the opening scene at the gallery, inter-cutting with the photographic studio, which brilliantly set up two essential themes; crime as art and photography as (at least partial) motive for crime. Credit must also be given to the production design of this episode. Production designer Jeff Tessler and his team must have had so much fun. Not only are they given the opportunity to design and construct two long-running sets (Poirot's new flat and Mrs Oliver's flat - both strikingly similar to the descriptions in Christie's stories), but the rooms in Shaitana's flat are really exquisitely done, suitably exotic and sufficiently flamboyant.
Locations include the 'Peacock House' / Debenham House in Holland Park (doubling as Shaitana's house - and also as Lord Edgware's residence in Lord Edgware Dies (2000)), Leighton House Museum (the interior functions as Shaitana's entrance hall), the Ham House Stables, Alexandra Court, 171-175 Queen's Gate, London (Mrs Oliver's apartment building - which would become the setting for Third Girl as well), Neal&Palmer, Piccadilly Arcade (where Poirot buys the stockings), and the Albert Memorial in London (see photos here).
Stephen McKeon's score is particularly effective in this episode, with a suitably mysterious atmosphere to it. Parts of it can be found on his website, e.g. 'Cat and Mouse' and 'Shadows and Light'. Also, I can once again reccomend the 2006 behind-the-scenes documentary of Series Ten - if you can get hold of it. There's plenty of interviews with the cast and crew.
Characters and actors
I have to mention the introduction of Zoë Wanamaker as Mrs Ariadne Oliver. You can read my blog post on the character for a more in-depth look, but suffice to say that this was an inspired bit of casting. She might not be quite as battleship-like as the character from the novel, but to me, she's absolutely perfect as Ariadne, and her coupling with David Suchet's Poirot is absolutely perfect. Also, it's a joy to see Poirot have someone to play off, after an entire series 'on his own'. Of the other guest actors, Alexander Siddig perfectly captures the Shaitana of the novel, and the fantastic Lesley Manville is particularly moving as Mrs Lorrimer (and I'm quite impressed by her delivery of the scripted bridge lines in the interview with Poirot! That's almost as impressive as Benedict Cumberbatch's fast-speaking Sherlock). But of course, having only four (five) suspects and four sleuths, all the actors have time to excel in their parts - and most of them do.