Script versus short story
The script stays more or less true to the short story. Obviously, most of Japp's retelling of the case is dramatised, but apart from that the structure is kept more or less intact. However, there are several added scenes and plot elements. First, there's an opening scene at a music hall in which Poirot, Hastings and Japp attends a magician's show (and Poirot reveals the trick!), which develops into a subplot in which Poirot learns magic tricks from "The Boy's Book of Conjouring" (slightly out of character, but it works, and it's no surprise that someone as meticulous as Poirot succeeds). Second, there are several scenes at a race course (derived, in part, from a reference to a 'race meeting at Entfield' in the short story), in which Hastings gets to display his affection for Italian race cars (building on the car interest from Series One). Third, there's the by now infamous "Parrot for Mr Poyrott" scene (which is amusing, but I've always considered that somewhat out of tune with Christie's stories). Fourth, Miss Lemon is added to the story (but she doesn't get much to do, apart from watching Poirot do his magic tricks and serve him his tisane). Fifth, Hastings gets to investigate on behalf of Poirot (resulting in a series of funny scenes between Fraser and Jackson), particularly a scene in which he attempts to break into the safe (on Poirot's instructions - somewhat reminiscent of a scene in Murder in Mesopotamia, in which Poirot tests the sound between two rooms). Sixth, two "clues" are added to the mix; the contents of Davenheim's bathroom cabinet (razors and shaving foam, evidence of his transformation), and Tchaikovsky's '1812 Overture' (used to camouflage the break-in of the safe). All in all, though, the adaptation works and stays fairly true to its source material, apart from possibly one too many comic relief scenes (for my taste, at least).
Directing, production design, locations, soundtrack
Andrew Grieve's direction is a delight to watch. There are several clever shots in this one. He utilises the beautiful location (the Davenheim house, actually 'Joldwynds' in Surrey, a house that would be used for The Theft of the Royal Ruby too). For instance, there's a wonderful shot from inside the house in which Mrs. Davenheim looks out of the main window and sees her husband disappear in the fog. Also, there's a nice shift between scenes when Lowen adjusts his moustache and Poirot adjusts his in Whitehaven. Other locations include Brooklands Museum (Surrey), which was used for the race course scene. The ghost-like soundtrack is perfect for the story (again, we have Fiachra Trench at work), and whoever came up with the idea of using the '1812 Overture' as a plot point deserves a pat on the back.
Actors and characters
It's nice to see the friendship between Poirot, Japp and Hastings develop over the course of this series. They attend shows together and have dinner at Whitehaven. Suchet gets quite a lot to play with (apparently, he did all the magic tricks himself). The house of cards is a nice reference to The Mysterious Affair at Styles and Three Act Tragedy. Of the guest actors, Mel Martin and Kenneth Colley, who play Mrs. and Mr. Davenheim, stand out. Colley almost manages to pull off the important disguise trick.