Script versus novel
Harcourt, who also scripted Murder on the Orient Express, is admirably faithful to the source material here. The most significant change, perhaps, is to set the adaptation in the late 1930s, directly preceding the Second World War. It's both a necessary and a wise move. It's necessary because of the production team's creative decision to keep Poirot in the Thirties, and it's wise, because the transition of an essentially 1960s spy novel into pre-war espionage is seamless. The setting is Dover, and Dover Castle is a perfect backdrop for this pre-war story (reminiscent of Foyle's War's Hastings at times). Another change that's also dictated by the series itself is the decision to change Colin Lamb's name to Lt. Colin Race and make him the son of Colonel Race. In the novel, Christie implies that Colin might be Superintendent Battle's son ('Lamb' is a cover), but since Battle was never introduced to the television series; he was deleted from Cards on the Table and replaced by Superintendent Wheeler. Now, Colonel Race was introduced to the viewers in Death on the Nile as an old and cherished friend of Poirot's. He should also have been in Cards on the Table, but actor James Fox was unavailable to reprise the role. To delete the fake identity and make Colin his son makes absolute sense, and I'm glad the scriptwriters pay attention to continuity every now and then.
As I've already implied, the adaptation is faithful to its source material. There are, however, several minor (and more significant) changes. Harcourt adds an opening scene in which we see Larkin (who has become Annabelle here) steal some documents from the navy base at Dover Castle. Race's girlfriend Fiona, who works there, follows Larkin and is eventually hit by a car (but she manages to write down the code, 'M 61' before she dies (like the secret agent in the novel). To give Race's investigation a personal touch is a clever move, as his professional judgement is at stake. Moreover, Harcourt adds a theatre performance attended by Poirot. It's one of Mrs Oliver's crime stories, starring Sven Hjerson. This scene is delightful for a number of reasons. We have a reference to Ariadne Oliver, who has really become Poirot's closest friend in these later years of his career. We get to see her detective, who in many ways is exactly as eccentric and 'foreign' as Poirot. Then there's the fact that Ariadne had been persuaded by Robin Upward to adapt one of her novels for the theatre in Mrs. McGinty's Dead (2008) - the actual production never began (for obvious reasons). But most importantly, the scene is a reference to the novel Dumb Witness and the short story adaptation 'Third Floor Flat', in which Hastings takes Poirot to the theatre to see a 'whodunnit'. Obviously, Poirot would be extremely bored (as indeed he is when he is interrupted by Colin Race in this adaptation).
By this point I should probably mention that Harcourt introduces Poirot much earlier than in the novel, and he is in a much more active 'investigative' mood here (in the novel, his valet George (who is sadly lacking from this adaptation, but he gets a mention, explains to Colin that he thinks his master might be getting depressed). This change is in keeping with the more recent adaptations, which have all introduced Poirot earlier than the novels. Of course, part of the fun of the novel is to see Poirot solve a case from his armchair (which he is famous for), but this has been referenced in several of the short story adaptations (most prominently in 'The Disappearance of Mr Davenheim'.
But let's list the main changes. Sheila Webb isn't Miss Pebmarsh's daughter (that was too unbelievable anyway). She does, however, have an affair with one of her clients, Professor Purdy. This wasn't in the novel, but it demonstrates her self-doubt and insecurity (that will eventually make Colin fall in love with her - they are both 'damaged goods'). Miss Pebmarsh works for the local photographer, Mr Wright (not as a teacher). We eventually discovered that she used the equipment available to her to photocopy the documents that were smuggled out of the Dover facility. Harcourt has added a scene in which Vice Admiral Hamling (a new character that sort of replaces Colonel Beck) discusses the coming of war with Poirot ('hellfire corner'). It's a lovely reference to Poirot's past in the Belgian police and as a war refugee. Mr. and Mrs. Ramsay have been removed (but a Mr. Mabbutt replaces them). Mrs. Curtin (the cleaning lady), Sheila's aunt and the Naughtons are removed, and the boys Ted and Bill have become two girls, Jenny and May. The Waterhouse siblings have become Jews (previously named 'Tuchmann', they fled Germany in the early 1930s). The interviews are generally a lot sillier than in the novel, and so is most of the action (but many fans will welcome the return of some humour to these adaptations). The interaction between Inspector Hardcastle and Poirot is particularly well done, reminiscent of the days with Japp.
There are essentially two threads to this story, with the murder in Miss Pebmarsh's house being one and the spy/war plot being the other. Apart from Pebmarsh's house, there's not much that links the two (which is partly the point of the novel). This has been expertly streamlined here, and the spy plot has been given a more sensible backstory here (all that 'secret offices in bookstores' from the novel is not very Christie). Here, Miss Pebmarsh wants to prevent a new war no matter what costs, because she lost her two sons in the First World War. Mr. Mabutt has a possibly more business-like approach to the treason, but both stories work well here. There's also a confrontation between these two and the Jewish siblings, in which Poirot delivers a powerful speech on the perils of occupation (once again, Harcourt manages to reference his Belgian past). The adaptation ends as Poirot brings the two lovers (Sheila and Colin) together, perfectly in keeping with his matchmaker hobby (Sad Cypress, Mrs. McGinty's Dead etc). All in all, Harcourt's script is a success. It manages to streamline a fairly second-rate Christie novel and make it a good adaptation.
Directing, production design, locations, soundtrack
Charlie Palmer's direction works well here. It doesn't draw too much attention to itself, but it's well executed. Jeff Tessler's production design is spotless as always (the Dover HQ and the different houses are decorative sets). The locations used include Dover Castle, Castle Hill Road, St. Margaret's Bay, Richmond Theatre dress circle bar, Thornhill Crescent in London (Wilbraham Crescent), St. Andrew's Church in Thornhill Square, Woburn Walk / 13 Duke's Rd in London (the shops and the secretarial bureau), The Sun Inn Bar in Richmond (the B&B Hardcastle recommends), The Churchill Hotel (Dover Seafront), Inner Temple in London (Rival's murder, also seen in Third Girl and possibly The Big Four), Fountain Court (Middle Temple) and Surrey County Council (the court scenes). See some photos here, here and here. Christian Henson's soundtrack is delightful here, with several very obvious hints to Gunning's theme tune, particularly in the opening sequence.
Characters and actors
Poirot shines in this adaptation, with a lot of time to investigate. I like the 'thinking mode' Suchet lets him enter (see, for instance, the scene at the theatre with Colin or the scene in his Dover hotel room). It's also nice to have a reference to George and something reminiscent of Japp and Captain Hastings in Hardcastle and Lt. Race (I even thought the ice cream scene reminded me of the end scene with Hastings, Japp and Miss Lemon in Peril at End House, but perhaps I'm just excited about their return for The Big Four). The references to Poirot's past (and his memories of war) are particularly delightful, too. Of the guest actors, Geoffrey Palmer provides the necessary gravitas as the Vice Admiral. Tom Burke (son of David 'Dr. Watson' Burke) and Jaime Winstone (daughter of Ray Winstone) are sufficiently 'damaged' as Colin and Sheila. But I particularly enjoyed the more joyful characters of Hardcastle (Phil Daniels), Merlina Rival (Frances Barber, who also played 'Lady Millicent' in 'The Veiled Lady') and Mrs. Hemmings (Beatie Edney, who also played Mary Cavendish in The Mysterious Affair at Styles). Oh, and John Thaw's daughter Abigail Thaw plays Rachel Waterhouse, so we have something of a collaboration between the Poirot, Holmes and Morse spheres here!