This episode was based on the novel Third Girl, first published in 1966. It was adapted for television by Peter Flannery (creator of the television adaptations (not the novels) George Gently), and directed by Dan Reed.
Script versus novel
Peter Flannery's task of adapting this novel for television series was not an easy one. The novel itself is significantly coloured by the Swinging Sixties atmosphere it is keen to portray (and was written in). Following the norm of the series, the novel had to be transported to a mid-1930s setting. Flannery partly succeeds. He makes several significant changes (the blog 'Sound Insights' points out that he 'maintains pretty much only Norma’s initial claim, the unmasked impostor and the imposter’s murderous co-conspiritor'). Some of the changes work well, and some don't. Most importantly, Norma's backstory is significantly altered. She is never given drugs as in the novel. Instead, her disoriented state is blamed on the trauma caused by her mother's suicide, which she witnessed at the age of seven (her mother was, unlike in the novel, Mary Restarick). SPOILER In fact, her mind is manipulated by Frances, who planted (and later removed) a knife from her room. I'm not entirely sure if this explanation works, and I still don't quite see why they couldn't have introduced a drug of sorts (the series has tackled drugs before, e.g. in 'The Affair at the Victory Ball' and Peril at End House). Moreover, three friends of Poirot are deleted from the plot; Miss Lemon, Mr Goby and Dr Stillingfleet. I know many fans were annoyed that Miss Lemon was deleted. To some extent, I agree that her presence would have been a nice addition. However, I think it's important to remember that the character of George (the valet) was deleted in the earlier adaptations, so to increase his presence at the cost of Miss Lemon's now in the later years (particularly considering that Poirot is semi-retired) seems fair. As to Mr Goby, he has been deleted from previous adaptations and his presence seems unnecessary here anyway. Dr Stillingfleet's presence would perhaps have been welcome, but Norma's future is secured by a different change (David Baker survives). A further (and significant) change was to replace the character of Louise Charpentier with a new character, Nanny Lavinia Seagram, who looked after Norma when she was younger. Her murder is committed in exactly the same way as Mary Restarick committed suicide (instead of being pushed out of a window like Louise Charpentier), but the motive for the murder is the same. SPOILER Also, Mary Restarick doesn't pretend to be Frances Cary (that would have been too much like the solution in One, Two, Buckle My Shoe). Instead, Frances Cary becomes Norma's half-sister (Norma's teacher Miss Battersby had had an affair with Andrew Restarick that resulted in Frances. She told her daughter about Robert Orwell, and Frances found a way to become his co-conspirator (she wanted to inherit her half-sister's fortune). Of minor changes, Mrs Oliver explains that she sent Norma to Poirot much earlier than in the novel; she also gets to search Nanny Seagram's flat (a delightful scene). Moreover, Sir Roderick wants Poirot to look into David's background rather than find secret papers. Also, Inspector Neele becomes Inspector Nelson (not sure for what reason), and a lovely end scene between Mrs Oliver and Poirot is added. All in all, then, this adaptation largely works, apart from the fact that the Sixties atmosphere is somewhat awkwardly adapted to a 1930s setting. Both the concept of the 'third girl' (which remains in the plot) and the influence of drugs (which doesn't) would probably have been better suited to a later decade. As it is, though, with the series firmly based in the 30s, I actually think Flannery's attempt is almost as good as it gets, considering the challenges.
Directing, production design, locations, soundtrack
The episode is competently directed by Dan Reed. I particularly like the opening sequence in Poirot's flat, with sweeping overview shots, as well as Mrs Oliver's chase scene later in the episode (brilliantly acted, too!). His use of overhead shots adds to the somewhat sinister atmosphere (and so does the colour grading). The production design is impressive as always, with special credit given to Jeff Tessler's design of the Whitehaven flat, which plays a central role in this episode. Stephen McKeon's soundtrack works well for the episode (for the most part), and it's nice to see Gunning's theme tune hinted at again.
Characters and actors
The friendship between Poirot and Mrs Oliver continues to develop - there are several scenes in this episode that are simply a delight to watch (watch out for Poirot's comment: "I assure you, this is how she is"). Zoë Wanamaker is her usual brilliant self. Of the guest actors, Jemima Rooper isn't half bad as Norma, which is impressive, considering the complexity of the character.