This episode was based on the short story 'The Lost Mine', first published in The Sketch in 1923. The screenplay is by Michael Baker and David Renwick, and the director was Edward Bennett
Script versus short story
The scriptwriters make some rather extensive additions and changes to the plot, which is fairly understandable, considering the fact that the story itself is very thin. Most importantly, the plot is shifted from past to present tense. Second, Japp and Sgt. Jameson from Murder in the Mews (replacing Inspector Miller from the short story), and Miss Lemon are added to the story. Third, several plot points are added, including the collapse of the Imperial Trust, Miss Lemon's stock speculation (!), a clue of discoloured cigarettes at the hotel (due to the blackened teeth), an entire subplot concerning Reginald Dyer (who is involved in some shady business in the short story but here becomes a slippery crook), an expanded back story for Lester (including a distressed wife), modern police work at Scotland Yard (see also the adaptation of Four and Twenty Blackbirds), a chase scene (of course), Poirot's bank account (which seems to be overdrawn, but is later revealed to miss a bill from Poirot's 'naval secrets case' - a reference to the unfilmed story "The Submarine Plans" perhaps?), and a game of Monopoly between Hastings and Poirot (a running joke throughout the episode). Fourth, Lester becomes an investment adviser (as opposed to a 'young bank clerk'), Fifth, Japp discovers the opium den through one of Dyer's contacts, and finds both Dyer and Lester down there. All in all, the story is changed almost beyond recognition. That, however, is not necessarily a bad thing. After all, the original story really only served one purpose, that is for Poirot to explain that Hastings shouldn't make speculative investments, so there was definitely a need for a more extensive plot to make it work as an adaptation. To a certain extent the changes work - and even make sense - so I suppose we should consider this adaptation a moderate success.
Directing, production design, locations, soundtrack
Bennett makes excellent use of the locations for this episode, particularly the recreated area of Chinatown. There's also some rather nice scenes at the bank. In Peter Haining's book on the series, Bennett describes the choice of location. 'We wanted an authentic look, and the Civic Centre (in Essex) is in very good condition. The interiors we used were just what we needed and very official looking, too. The council offices made for plush banker's rooms and the whole effect was Thirties to a tee' (p. 49). Columbia Road and Ezra Street in Bethnal Green became a replica of Chinatown. Once again, the attention to detail is extraordinary, and the production designers have done such a good job. For location photos, see this link. The soundtrack is good and somewhat oriental in tone. I think it was done by Fiachra Trench, who stepped in for Gunning through most of Series Two. One of the scores, for the chase scene, will be reused in The Kidnapped Prime Minister.
Characters and actors
This episode introduces Hastings's hunch for bad investments (giving the wrong advice to Miss Lemon), a character trait we will see in several future episodes (see, for instance, Lord Edgware Dies). There aren't really any stand-outs as far as guest actors are concerned. Anthony Bate does a decent job at portraying Lord Pearson, but none of the performances are that memorable, really.