The novel is set some time during the Cold War, and features a spy fiction sub-plot, narrated in first person by Colin Lamb, a British Intelligence agent trying to track down a spy who has been passing information to the enemy. Colin's investigation brought him to the scene of a rather peculiar murder case, in which an unknown man was found stabbed in the house of Miss Pebmarsh, a blind woman. The man carries no identification with him, and no one seem to recognise him. His body was discovered by Sheila Webb, a typist who was apparently hired by Miss Pebmarsh, although the latter denies making such appointment. Colin then brings the case to Poirot, who agrees to investigate the murder through Colin's accounts.
In 2010, ITV adapted the story for the 12th season of Poirot.
This novel provides examples of the following:
- Couple Theme Naming: The name on the dead man's fake card is "R. H. Curry", and the woman who says she is his widow is called Merlina Rival. Poirot figures out she is the murderer's accomplice, because he immediately recalls the Somerset village of Curry Rival and realizes the murderer, while printing the card, subconsciously picked a name to match Mrs. Rival's.
- Crazy Cat Lady: Mrs. Hemming, occupant of 20 Wilbraham Crescent, owns about 20 cats, and can't seem to talk about anything but cats.
- Decoy Protagonist: Colin may initially seem like the protagonist, being the "outsider" who stumbles into the crime scene and was dragged into the subsequent investigation. However, his role in the case is secondary to Hardcastle. Even the suspects they interrogate see him as nothing more than Hardcastle's extra hand... though as Colin works in espionage, that's likely the impression he wanted them to get.
- Dirty Commies: Colin is in the neighborhood to investigate possible Soviet agents transmitting intelligence back to Moscow.
- Everyone Can See It: Inspector Hardcastle, Poirot, and Colonel Beck (Colin's superior) all realise that Colin has fallen hard for Sheila due to his obstinate conviction that Sheila is innocent. Colin himself insists (truthfully) that he has a perfectly logical reason for believing her innocence, and does not admit that he has less-than-professional feelings towards her until halfway through the book.
- First-Person Peripheral Narrator: Zigzagged. Colin is only tangentially involved in the murder investigation, and barely makes any contribution in the interrogation of suspects. However, he's an undisputed lead of his own subplot. On the other hand, the first-person accounts only make up less than half of the actual narrative, as the novel is mostly written in third person.
- Hero of Another Story: Despite being the (occasional) narrator of the novel, and the man whose accounts of the investigation lead Poirot to the solution of the mystery, Colin's main concern is to find The Mole who has been selling British secrets to the Soviets, a spy subplot that is somewhat glossed over in the actual events of the books.
- Impersonation-Exclusive Character: Mrs. Valerie Bland is revealed to be Mrs. Hilda Bland, her husband's second wife, posing as the first one to get her inheritance. The only things we therefore know about the real Valerie was that she came from a rich Canadian family and had really poor judgment of men, considering her choice of husband.
- Love at First Sight: Sheila Webb runs into Colin Lamb, and the latter immediately decides she is his girl.
- Luke, I Am Your Father: Miss Pebmarsh is revealed to be Sheila's Missing Mom, although Sheila is never told of this fact.
- Motor Mouth: Edna Brent is unable to stop talking, to the amusement (and sometimes annoyance) of her co-workers.
- Parental Abandonment: Sheila lives with her aunt, who has told her that her parents are dead. In truth, she's an illegitimate child. Her mother, Mrs. Lawton's sister, gave her up because she was an ambitious woman who didn't want a child to hinder her career.
- Phone-In Detective: Poirot never sets foot at the crime scene, nor does he actively take part in the investigation. He makes all his deductions based on Colin's verbal reports.
- Red Herring:
- The titular clocks are not as important to the plot as one would believe them to be, and were in fact placed there deliberately to confuse things.
- While it turns out, coincidentally, to be connected to the murder plot, number 61 is a red herring in Colin's espionage investigation. He read the clue upside down, and it was tipping him off to #19 all along.
- Ripped from the Headlines: Inverted in-universe. The killer used an unpublished mystery novel manuscript as the basis of their murder scheme.
- Shout-Out: Colin visits a girl who lives across the street from the murder scene who, confined to the apartment with a broken leg, passes the time watching her neighbors. Sound familiar?
- 6 Is 9: Colin begins the story trying to find number 61 on a notoriously confusing street—based on a handwritten tip off—when he runs into a murder mystery at number 19. He doesn't connect the dots until the very end of the mystery, so maybe it's for the best that he decides to quit the intelligence services.
- Two Lines, No Waiting: Two separate investigations are taking place in Wilbraham Crescent. One is of the murder scene in Miss Pebmarsh's home, and one is an espionage plot where one of the occupants of the neighbourhood is believed to have been passing national information to the enemy. Aside from the fact that they take place in the same location and Colin is involved in both investigations, the two plots are not actually related to one another. Though it is worth noting the address Colin initially wants to investigate houses the murderer (well, some of them), and the house where the body was found is home to the spy Colin is searching for. Also, while it's never explicitly spelled out, Colin has his "Eureka!" Moment when he sees Hardcastle writing on some upside-down stationery — because he should have been investigating number 19, not 61, all along.