Script versus short story
The original story that served as a basis for this episode is very slight, so it obviously needed some embellishment to work as an adaptation. It is a testament to Horowitz, then, that most additions seem perfectly natural, as if they had been part of the original story in the first place. Not surprising, really, when you take into account that Horowitz went on to write for Midsomer Murders, the brilliant Foyle's War as well as a bunch of other crime series - in addition to novels for young adults, and new Sherlock Holmes novels. A scriptwriter genius in the making, one could perhaps say. Now, back to the adaptation. First, he adds an opening scene in Threadneedle Street (the exact location mentioned in the short story), in which Mr. Shaw is nearly run over by a sports car. Second, Horowitz changes the Olympia into the maiden voyage of the Queen Mary (in keeping with the series' inclusion of real-life 1930s events), and Poirot and Hastings join Ridgeway on the journey. Third, Mr. Shaw and Mr. Vavasour consult Poirot, not Esmee, and they do so before the robbery. Fourth, Mr. Shaw is himself supposed to be travelling with the bonds, but he is almost killed by strychnine poisoning (this is the third episode in a row with a death or near-death from strychnine!). Fifth, Inspector McNeil becomes McNeil, Head of Security at the bank, and Esmee becomes a secretary of Mr. Vavasour (but remains Ridgeway's girlfriend, too). Sixth, a second culprit is introduced; Nurse Long aka Miranda Brooks. She nurses the (apparently) ill Mr. Shaw, but she also drives the sports car, travels on the Queen Mary (instead of Shaw), breaks into the box and throws the package over board. She also serves as a love interest for poor Hastings, but is in fact Shaw's wife-to-be. In the end, it's a habit that blows her cover; she grasps for her nurse uniform watch when Poirot asks for the time. Seventh, Ridgeway is in money trouble from all his gambling (in the short story he has never had debts in his life), thus providing him with a potential motive. Most of Horowitz's changes are admirable. He broadens the list of suspects, creates possible motives and makes use of classic Christie plot elements like strychnine poisoning and the well-used no-one-looks-at-a-servant/nurse/maid-trick.
Directing, production design, locations, soundtrack
Grieve's direction is a delight to watch. For instance, in the opening scene at Threadneedle Street station, he is showcasing the routine-like life of bank clerks on the tube (that will eventually become part of the motive - "Prison can't be much worse than 25 years at the London and Scottish Bank"). Moreover, there's a nice shot of the coffee cup on a trolley, indirectly alluding to the sea voyage to come, and a subsequent miniature toy cruise ship on a pond in the park. Also, he makes good use of the newsreel footage of the Queen Mary and the set, almost convincing the viewer that it's the real thing. The production design is, as always, top-notch. I haven't been able to track down any locations for this one, apart from the Threadneedle Street entrance, but I have a feeling that parts of the bank interiors are from the Freemason's Hall in London, a location used for several Poirot episodes. The soundtrack is effective, but not much more. It has not been released.
Actors and characters
Thankfully, Horowitz alludes to Poirot's seasickness. Having Poirot on a cruise ship for a crossing as long as this is almost entirely out of character, but he has travelled by boat in previous episodes, so it's passable. He even explains the source of this dislike of the sea; it is apparently a result of the journey from Belgium twenty years ago (love that little reference to Poirot's back story). Miss Lemon is added to the plot, of course, but her role hardly seems intrusive - she remains firmly in the background. Hastings gets to recognise the car that attempts to kill Mr. Shaw (a reference to his car enthusiasm). Of the guest actors, there are no real standouts, but Lizzy McInnerny (Nurse/Brooks) managed to fool me the first time I watched this episode, I seem to recall.