The character of Lieutenant Colin Race is the son of Colonel Johnny Race, who appeared in Death on the Nile (2004). Poirot asks how the Colonel is doing while they are in the theatre bar. In the original novel, he uses the pseudonym Colin Lamb, and is the son of Superintendent Battle - a character cut from the adaptation of Cards on the Table (2006).
Actor Tom Burke's (Colin Race) parents previously appeared in Poirot episodes; David Burke in "Hickory Dickory Dock" and Anna Calder-Marshall in "After the Funeral."
The character name "Nora Brent" is also used in the Agatha Christie episode Nemesis. However, in the book The Clocks on which this episode is based her name is Edna, not Nora.
The original novel "The Clocks" was published in 1963, and featured a Cold War-themed spy story. The spy ring of the novel is affiliated with the Soviet Union. The television adaptation moves the setting to the late 1930s, and the spy ring featured is affiliated with Nazi Germany.