The episode's MacGuffin is changed from harbor defense plans to be sold to Japanese buyers to submarine plans to be sold to Italian buyers.
The Robinsons are chosen as Carla Romero's subletter solely because they share a name with her pseudonym. In the original story the real Mrs Robinson was a redhead like Elsa Hardt (and still is in the series), while here Carla Romero is a brunette.
Adaptation Name Change: Downplayed. Elsa Hart (note the lack of a D as in the original) is a stage name for the singer Carla Romero.
Chekhov's Gun: A rather literal example. Poirot chances upon a loaded gun and takes its bullets out. When the same gun is later used against him, he decides to dare the Mafia assassin wielding it to shoot him, knowing full well that said gun wasn't loaded anymore.
Cowboy Cop: Discussed. Poirot expresses his disappointment at police officers who shoot first and ask questions later after watching a film featuring one.
Eagleland: FBI Agent Burt is a downplayed Type 2 — loud and obnoxious, but not ungrateful. When Poirot expresses his disappointment to Japp regarding their American liaison, Japp can't help but agree.
Poirot: "The liaison transatlantique does not live up to its expectations?"
Japp: "Basically, to use a technical term, it's a disaster."
Entertainingly Wrong: FBI Special Agent Burt insists there is no such thing as the Mafia (per official FBI policy) right up until a Mafia hit man holds him at gunpoint.
Eskimos Aren't Real: Agent Burt refuses to believe in organised crime, insisting that the Mafia only exist in sensational fiction.
Foreshadowing: The episode starts with Poirot and friends watching a movie featuring a gung-ho American cop. Surely enough, they later work with a gung-ho American law enforcement officer.
Hastings' friend Parker met the Robinsons at a theatre bar - they had an almost-identical drink ticket to him, and drank his drinks by mistake. In the end, the Mafia assassin is caught in the Robinsons' flat - they had the same name on the lease as his target, and he attacked them by mistake.
Show Within a Show: The episode starts with Poirot, Hastings and Japp watching the film G Men (1935).
Significant Name Overlap: The Robinsons got their new flat for a cheap price because they shared the name of the original tenant who sublet it to them, a "Mrs Robinson", which turns out to be an assumed name for Carla Romero. This is so that the latter could throw off the men on her trail by making it appear she never had the place.
This Looks Like a Job for Aquaman: Miss Lemon gets to provide major assistance in the case when she interviews Elsa Hart in the guise of a reporter. Given that Elsa made herself scarce to men looking for her, it's easy to see why she'd let her guard down when talking to another woman.
We Need a Distraction: Poirot needs a distraction to keep the Robinsons occupied while the former surreptitiously investigates the flat. He drafts Hastings into keeping them occupied by having him strike up a conversation with them.
You Wouldn't Shoot Me: When a Mafia assassin attempts to kill Carla Romero and her husband for the murder of Luigi Valdarno, Poirot steps in their way and tells his colleagues that the assassin would not dare shoot him. The assassin pulls out Hastings's gun and warns him that if he takes one more step, then he is finished. Poirot acts all like "Try me!" and takes one step. The assassin prepares to shoot him and... the gun clicks a few times before he realizes that the gun has no bullets. Poirot then shows him the bullets that he had taken out of the gun earlier while he and Hastings were tracking down the assassin in the Robinsons' apartment.
