'The Submarine Plans', a short story published in 1923, was later expanded by Christie into the story 'The Incredible Theft', first published in the UK in 1937. It was the latter short story that was adapted for the first series of Agatha Christie's Poirot. The screenplay was jointly written by David Reid and Clive Exton. The director was Edward Bennett.
Script versus short story
The script stays more or less true to one of Christie's longest short stories, with certain important additions and changes. First, in terms of characters, Mayfield has a wife in this adaptation, Lady Mayfield, who only calls herself 'Lady' because her father was an earl (Mayfield is not a Lord in this adaptation, he is simply credited as 'Tommy Mayfield' (changed from Charles in the short story). Mrs. Macatta, M.P. is deleted here, which is hardly surprising considering the small role she has in the source material (and, in any case, Poirot and Lady Mayfield fill out her absence, in a way). Sir George Carrington seems to have been changed from an Air Marshal to a senior civil servant of some kind, as far as I can make out. Moreover, Poirot's little family is added (Hastings, Japp, Lemon). The addition of Hastings makes sense, in a way, since he was in the the first version of the short story ('The Submarine Plans'). Miss Lemon gets to take a telephone call from Lady Mayfield, which makes sort of sense. Japp has been asked by Carrington to stay at the local pub, which I guess is almost believable. Also, the whole incident with Mrs. Vanderlyn's maid, Carlile and Reggie Carrington has been excluded, which I must admit is difficult to understand. Yes, the whole thing was a bit thin, but at least it gave time for someone to enter the office and steal the plans, theoretically speaking. In the adaptation, Carlile is adamant that he never left the room, which makes it a lot more unbelievable than anyone could have taken the plans...
In a way, I've already touched upon the main changes. First, we have an opening sequence with a fighter plane (looking suspiciously like a Spitfire), and a conversation between Mayfield and Carrington (parts of which took place after dinner in the source material). Second, there's a scene in which Poirot is polishing his patent leather boots and instructing Hastings on the subject of women (apparently, Hastings is seeing this young 'student of architecture'!). Third, there's the anonymous woman asking for Poirot (and angering Miss Lemon and her filing system), who after a meeting with Poirot reveals herself as Lady Mayfield. Exactly how she then manages to get him to dinner on the night the plans are stolen, I don't quite understand. Supposedly, he is her guest at dinner; 'the famous detective' does, after all, sometimes accept dinner invitations. It certainly wouldn't make sense for him to be invited by the Mayfields in case Mayfield's plan goes wrong. Finally, there's the added police involvement, which is sort of out of place (considering they were so anxious to avoid a scandal), but at the same time highly necessary (why else could they demand to search Mrs. Vanderlyn?). The police involvement also allows for a search scene in which Japp is fascinated by Mrs. Vanderlyn's silhouette in the frosty glass of a door (she is descibed as having a 'seductive kind of beauty' in the short story). There's also a humorous subplot of Hastings and Japp sharing a bed at the local pub (!) and a rather ridiculous chase scene with Poirot and Hastings following Mrs. Vanderlyn in a police car. Largely, however, the changes work, and the essentials of the story are kept intact.
Directing, production design, locations, soundtrack
Bennett's direction is competent and suits the story well. He is helped by some nice locations, for instance the vintage plane in the opening scene and the Penguin Pool of the London Zoo (see Joan Street's location site). Other locations include a private country house in Surrey doubling as the Mayfield residence and the area around Tilford providing country roads for the chase scene (see the location site for photos). The soundtrack for this episode is quite memorable, particularly what seems to be Mrs. Vanderlyn's theme (sadly not released by Gunning). The score from The Adventure of the Clapham Cook, 'To the Lakes', released on CD, here makes a reappearance for the country house setting.
Actors and characters
Poirot (or rather, David Suchet) gets to demonstrate the meticulous care he invests in his wardrobe (as he did in the first ever episode and will do in future episodes). Miss Lemon's filing system (mentioned in The Adventure of Johnny Waverly) gets a reference, and Hastings gets to help a police constable with a car and drive a police car in the car chase (once again demonstrating his love for and interest in cars).
The guest actors all do a nice job at portraying the different characters, but obviously, Carmen Du Sautoy as Mrs. Vanderlyn, is the one you will really remember, although John Stride and Ciaran Madden (playing the Mayfields) stand out as well.