This episode was based on the novel Dead Man's Folly, first published in 1956. It was adapted by Nick Dear (by now one of the 'regulars'), and directed by Tom Vaughan.
Script versus novel
Nick Dear's script is a very faithful retelling of the novel. Certain sections are moved around, some have a slightly different setting, and some sections are shortened down (especially the interviews), but most of the action is kept intact. Let's look at some of the changes. Obviously, the time setting has bee changed from post-war 1950s to pre-war 1930s. This doesn't manifest itself in any particular changes, apart from the deaths of Mrs Folliat's sons. Moreover, Dear adds an opening sequence that takes place a year before the investigation. The incident is based on conversations with Mrs Folliat and Merdell later in the novel. These opening sequences from the past that will later have an impact on the plot have become a norm on these productions over the years; a number of episodes have the same addition. Furthermore, the telephone call from Mrs Oliver in the opening chapter is removed, including Miss Lemon and the Whitehaven setting. Miss Lemon's absence makes sense, both because this episode isn't explicitly set after The Big Four, and because Poirot is in semi-retirement at this point of his career. George the valet could have made an appearance, but they probably decided not to include him because of availability issues or costs. Instead, Poirot has received a telegram from Mrs Oliver and is on his way from the station when we first see him. Some characters are deleted in the subsequent sections, including the Mastertons (though Mrs Masterton becomes Warburton's wife, and Warburton becomes a Member of Parliament), Sergeant Cottrell (his lines are given to Hoskins instead), the Chief Constable, and Mrs and Mr Tucker (Marlene's parents). None of these deletions really impact the story, and they are probably all a result of time constraints rather than creative decisions. A subplot involving Alec Legge and a man in a turtle-patterned shirt is deleted (probably due to time constraints, or possibly the fact that it doesn't really add anything to the plot). The incident in the camellia garden with Mrs Oliver and Poirot is deleted, and so is the police re-enactment of the possible drowning of Hattie.
Finally, the ending is changed. The setting from the denouement is changed from Folliat's lodge to the boathouse (they seem to have avoided the lodge throughout - I wonder if the location was unavailable or didn't suit the period setting?). Also, in the novel, the fate of the Mrs Folliat and her son is left open ('Will you leave me alone now? There are some things that one has to face quite alone...'). Here, Mrs Folliat asks Poirot to allow her to meet her son before she is arrested. He allows it, 'as a courtesy from an old gentleman'. She goes to James's study and tells him to do exactly what she tells him to do, for once in his life. Outside, two shots are heard, and they presumably commit a murder-suicide. Poirot seems to approve of this with the final word of the episode: 'Bon'. The new ending is intriguing. It gives the Folliats a more explicit fate, but we are not told who killed whom (reminiscent of Elephants Can Remember). Also, it's interesting to view Poirot's changed sense of justice since his encounter with the culprits in Murder on the Orient Express. The decision he had to make there has obviously affected his sense of justice (although he has 'allowed' suicides before - Peril at End House, The Hollow etc).
All in all, Dear has done an excellent job. The script is very faithful to its source material. He must know Christie pretty much inside out by now, having adapted a total of six episodes. That doesn't come close to Clive Exton, but his adaptations have generally done justice to the novels they were based on (possibly apart from the ending of Cards on the Table).
Direction, production design, locations, soundtrack
Tom Vaughan's direction really suits the atmosphere of the story and the location. He utilises the garden, the boathouse, and the woods to their utmost potential. They almost become a character of their own, helped along by the crows in the trees (reminiscent of The Tragedy at Marsdon Manor). The garden fête scene feels slightly rushed, but it does convey the hustle and bustle of the event. The production team have done an excellent job with the fête and the particularly colourful costumes in this episode. The main location used was Greenway, Agatha Christie's holiday home. It's a beautiful setting, and it really affects the way the story progresses. The house becomes a character of its own. Christian Henson's soundtrack works well for the episode (notice the minute hints to the theme tune every now and then). Some might find the muted brass instruments a bit too much, but I think they work for the atmosphere the adaptation is trying to create.
Characters and actors
Poirot is generally quite displeased with his skills this time around. That's partly based on the novel, but certain minute references are added to his 'grey cells' slowing down. Then there's his changing sense of justice, as evidenced in the end scene. It will be interesting to view this episode again when all 70 episodes have aired and consider the development of his sense of justice and morality. Of course, plenty of Poirot's eccentricities are added. He 'twirls his moustache to a ferocious couple of points' (the sentence, taken from the novel, was even a scene description in the script!), he struggles with the countryside and walking around in the woods, and he takes an instant dislike to the students in shorts. Also, there's a particularly funny scene with a large Kewpie doll, taken straight from the novel. Apparently, the scene was not intended to be included in the script, but Suchet asked for its inclusion (which reminds me of the scene with the marrow he insisted on for The Murder of Roger Ackroyd). The interaction between Suchet and Wanamaker is as brilliant as ever. Ariadne's incoherent police interview reminded me of the peacock scene from Third Girl. The afternoon tea between them in London was a nice addition. I particularly enjoyed the fact that Poirot called Ariadne back to Nasse by a telegram, the exact same method and the exact same meeting place (the battery). It highlighted the sense of humour between them. I only wish they had included the tiny reference to Hastings in that scene (but that's a minor complaint). The final exchange between them, on their two favourite methods (deduction and intuition) was also a nice touch.
Of the guest actors, Sinead Cusack stood out as Mrs Folliat. Sean Pertwee did a good job as Sir George, and several of the actors in minor roles suited their characters perfectly. The lack of an Italian accent (or small grammatical mistakes) in Stephanie Leonidas' Hattie was something of a plot hole. Similarly, Fransesca Zoutewelle's Dutch accent seemed a bit overdone, but then again the point of her character is to stand out as 'foreign', so perhaps it was necessary.