Script versus short story
Horowitz does an excellent job of bringing this short story to life. There are several important additions to the storyline, like Poirot's observations on marriage (a passage I feel sure comes from one of the Christie stories, but I can't remember which, if any), Japp's concern for his job, Poirot and Rossakoff's excursions to museums and parks, Hastings's and Miss Lemon's interviews with the other suspects (quite nicely done), their concern for Poirot (and themselves) in the face of change (aka Rossakoff), and the added red herring of a tramp seen at the scene of the crime. Horowitz also makes some clever changes to the two clues, making the gloves small (so that they can suit both men and women) and the engraved 'B.P.' represent both Bernard Parker (like in the story) and Lady Runcorn (whose maiden name, it is revealed, was Beatrice Palmeston). Of course, the love story is greatly expanded (as Poirot and Rossakoff are almost allowed to express their admiration for each other), but that certainly works, and, I would argue, is completely necessary to broaden Poirot's character profile a little. The denouement is also significantly changed to allow Rossakoff to escape (in the short story it's never explained how Poirot lets her off). However, for those who argue that this is an example of Poirot light-hartedly letting go of his principles (as oppose to the adaptation of MOTOE), I would just like to point out that Poirot has a rather significant exchange with Miss Lemon in one of the final scenes:
'I have from Inspector Japp the reports from the jewellery thefts. (silence) This work, Miss Lemon, it is not so straight-forward' (silence)Also, there's the fact that he employs the private detectives Redfern & Blake (a nice homage to other detectives Poirot makes use of in his later career), not only to lead Hastings and Japp off the scent, but also to ensure that the Countess leaces the country and is not involved in a fourth robbery, so one might say he ensures that the crimes will end (in Britain, at least). In sum, then, Horowitz has made an excellent adaptation of the story, and one of my personal favourites.
Directing, production design, locations, soundtrack
Piddington makes good use of the different location. For instance, the opening scene in which the Countess emerges from the train in the fog as some mysterious figure and then to leave on the same trend in the end scene, fully visible and characterised. The attention to detail in the many sets used for this episode is impressive. Locations include 'Shrubs Wood' in Chalfont St. Giles (Hardman's house - the location would later be used for Alistair Blunt's house in One, Two Buckle My Shoe), the grounds of Englefield House, Berkshire and Senate House, University College London (the art gallery). Finally, of course, there's Christopher Gunning's exquisite score for the episode, "The Double Clue" (available on his latest CD release). It sets the tone for the episode from the very beginning, and the subtle Russian influences make his score perfect for the task of conveying the importance of Poirot's greatest love.
Actors and characters
It's very nice to see a personal and emotional side of Poirot, and of course get references to his early years in England again (the conversation with the countess), and his thoughts on foreignness. Also, Pauline Moran's acting in this one is just wonderful. The way she delivers her lines, particularly in her conversation with Hastings on their future ("What about you?" "I don't want to talk about it...") and with Poirot in one of the final scenes ("Nothing, M. Poirot") so perfectly captures the emotional bond they've always intended between her and Poirot. She is absolutely devastated that he is taken in by another woman - and yet cannot bring herself to tell him anything. Of course, none of this is Christie, but it's so expertly and emotionally executed I am perfectly willing to accept that deviation. Also, of course, Horowitz adds Hastings's dream of farming in South America (which he'll also refer to in Yellow Iris). The way Horowitz makes references to everything - from historical events, previous episodes and Christie's stories is simply admirable. (As a chronology fanatic, though, I can't help but point out that he should have paid a tiny bit better attention to his time references...)
Of the guest actors, Kika Markham is obviously the main standout here. She is simply magnificent. Of course, I can agree with other fans who claim that her portrayal of the character is quite far from the character as written. Still, that has never stopped this production team before (see Miss Lemon or Japp, for instance), and I do think it works rather well for this particular adaptation. In any case, she'll be a tough act to follow in the adaptation of The Labours of Hercules, even if I have faith in Orla Brady (who is reportedly taking over the role). Also, I would like to draw attention to David Bamber (wonderful character actor) who does a really nice job with Bernard Parker.
As an aside, Kika Markham had several ideas of her own that she brought to the portrayal of Countess Rossakoff. In Peter Haining's book on the series, she explains: 'In the original stories about the countess she is described as the one great love in his life. For that reason I thought it would be natural for me to put my hand on his shoulder and then perhaps kiss him on the cheek - but I was told that it wasn't allowed. I did get to kiss him on the forehead, though, and that was only because I discovered it's how Russians behave when they are saying goodbye to someone close' (p. 19).