Hastings is present in the episode while he was absent in the original story.
Poirot actually meets Amelia Barrowby in the episode, where she gives him a cryptic clue before her demise.
Artistic License – Traditional Christianity: It's a plot point that Katrina is a devout Orthodox Christian. However, she is shown kissing the icons' images on their lips, which is a Catholic rather than Orthodox tradition. Orthodox Christians kiss the icons on their hands or, more rarely, on their feetnote If it's an icon of Christ depicted fully, such as the Crucifixion. or hair note For example, if it's a copy of the Image of Edessa., but never on the lips.
Better to Die than Be Killed: Subverted. Mary Delafontaine drinks from a bottle labeled "Weed Killer" so that she'd die of poisoning rather than be arrested and tried for murder... but it turns out that the bottle only contained regular alcohol.
Bilingual Bonus: Katrina and Nicholai's conversation early in the episode is left unsubtitled to conceal the fact that their dialogue is ultimately innocuous.
Busman's Holiday: Poirot's involvement in the case is due to his attendance of the Chelsea Flower Show, where a rose is named after him. Here he meets Amelia Barrowby, correctly fearing a threat on her life.
Dirty Communists: Subverted. Henry Delafontaine thinks Katrina is one because of her Russian background and her secretive nature, but she turns out to be anything but. Katrina's lover is suggested to be one at the beginning, but it soon becomes evident that h's more than he seems.
Discouraging Concealment: Henry Delafontaine hides his alcohol stash by labeling the bottle "Weed Killer" to prevent anyone else from drinking it. This later becomes important when his wife Mary takes a swig from it to prevent herself from being arrested; instead of being poisoned she's disappointed when she realizes it's just regular alcohol.
Heroic Russian Émigré: Katrina is revealed to be one due to having an altar with Orthodox icons and a picture of Tsar Nicholas II and later fleeing to the Russian Church in London for sanctuary. Downplayed in that she's shown to be acting rather suspiciously, but she turns out to be innocent in the end, and her beau works at the Soviet embassy.
Inheritance Murder: The Delafontaines murdered Amelia, Mary's aunt, and pinned the murder on her attendant Katrina for two reasons. First was that Amelia had left her fortune to Katrina, leaving them with next to nothing, and second was that they feared their embezzlement of Amelia's fortune on securities and pocketing the profits would be revealed. Amelia was aware of the latter, which was why the Delafontaines resorted to killing her.
Mistaken for Evidence: Ms Barrowby gives Poirot an empty packet of Catherine the Great stock seeds as a secret hint toward which of her household she suspects to be plotting against her. The name of the variety appears to point toward her companion, Katrina, until Poirot realises that the name is a coincidence, and the *species* was the hint - Ms Barrowby had discovered that her niece was stealing her money to speculate on stocks.
Multitasked Conversation: Mr. Harrison, Amelia's solicitor, reveals the beneficiary of Amelia's will using the horses they're inspecting as a cover, allowing them to discuss details without any third party being the wiser.
Japp reveals himself to be a flower enthusiast, enthusiastically reading a show catalogue and making jokes about the comparative merits of different roses.
Poirot is accused of wearing perfume, but protests that he is wearing a "manly cologne" that he got from Geo. F Trumper (a real barber and maker of men's cologne and shaving products). From the others' reactions, it evidently has a feminine scent.
Sick Episode: Poor Hastings is saddled with what he thinks is hay fever during the episode; it doesn't help matters that he goes to a botanical garden with Poirot. It's later revealed to be an allergy to Poirot's perfume.
Stealth Insult: Ms Barrowby's solicitor can't formally reveal which of her household has inherited her estate, so he hints by comparing them to the ponies in a show. Katrina is the filly, Mary is the mare, and Henry is the gelding.
Stealth Pun: Poirot notices that the Delafontaines' garden beds are lined with shells, and then finds a discarded bell buried in the dirt. He compares them to the titular nursery rhyme's silver bells and cockle shells... and then he meets the family's pretty maid.
Spy Speak: Subverted. Early in the episode Katrina and Nicholai appear to be discussing espionage in Russian at the Soviet Embassy. It turns out they were discussing something more innocent — likely relationship-related matters.
Working the Same Case: Immediately after their first visit to the Barrowby home, Poirot and Miss Lemon learn that Scotland Yard is already investigating, having been tipped off by Miss Barrowby's doctor.
