Script versus novel
Exton's script stays impressively close to the source material, as has become the norm by this point, but he makes some understandable changes. The setting is obviously moved from 1940 to the mid-30s (August 1937 to be exact), but the novel never explicitly states the year, so that's not really a change. He adds an opening sequence set twelve years earlier in India (including scenes with Blunt, Gerda and Sainsbury Seale, and a theatre production of Shakespeare's Much Ado About Nothing (Sainsbury Seale mentions in the novel that she was part of a production of Shakespeare's As You Like It, so this addition seems very sensible - and it's a good example of Exton's faithfulness to Christie's written word). The other dentist, Mr. Reilly, and his patients, are removed (they were nothing more than red herrings anyway, so it's an acceptable way of shortening the storyline). The entire Secret Service / Mr. Barnes / Mr. Chapman subplot is wisely removed (that seemed way too unbelievable). Poirot's valet George is obviously removed (he hadn't been introduced by this point in the series' chronology), and so is Mrs. Chapman's neighbour Mrs. Merton (in accordance with the deletion of the spy subplot). There's an added scene with a different Mr. and Mrs. Chapman, which actually provides Poirot with the idea for a clue (marriage certificates - we see him visit Somerset House afterwards). The attempts at Blunt's life are sensibly removed (again, unnecessary since the whole spy plot is removed). Helen/Gerda becomes Blunt's secretary rather than cousin (a sensible change) and she is the one who 'catches' Frank in the garden with the gun, since Mr. Raikes and his link to Miss Olivera has been removed. Finally, Mrs. Adams, a friend of the real Miss Sainsbury Seale is removed (she was unnecessary anyway). There are also some minor changes and obviously some parts of dialogue are shortened down or deleted, but mainly, the plot is kept intact and the adaptation is faithful to its source material. (P.S. I just want to point out that Exton manages to keep Poirot's denouement almost intact, word by word. That is quite an achievement.)
Directing, production design, locations, soundtrack
Devenish's direction is competent, and he does a decent job of trying to bring out the nursery rhyme connection in the opening scenes outside Morley's office, together with Gunning's soundtrack (which can be found on the CD). The production design is as good as always (notice the homage to artist Tamara de Lempicka in the painting in the board room), and the locations work well. They include Lichfield Court, Richmond, Surrey, 'Shrubs Wood' in Chalfont St. Giles (Blunt's country house - previously seen as Mr. Hardman's house in 'The Double Clue'), and a building in Harley Street.
Actors and characters
There's some nice characterisation bits here, including Poirot's continued fear of dentist visits (as seen in several previous episodes) and Japp doing his garden in Isleworth. As to the guest actors, Joanna Phillips-Lane (Gerda / Helen / "Sainsbury Seale") obviously stands out, almost managing to pull off an extremely difficult bluff for first-time viewers. And, of course, it's nice to see Christopher Eccleston (Frank Carter) of Doctor Who fame.