Three of the stories in the original "Partners in Crime" mystery story anthology were not made into TV shows; they were "The Adventure of the Sinister Stranger", "Blindman's Buff" and "The Man Who was No. 16". These stories comprise an ongoing case that spans the anthology. The introductory story, "A Fairy in the Flat" not only has the Beresfords asked to take over The International Detective Agency, but reveals that the agency's former manager, Theodore Blunt, was a part of a spy ring, and the Beresfords are tasked with intercepting coded messages. The three unadapted stories find the Beresfords threatened by various spies and eventually, discovering the identity of agent No. 16. The book ends with them closing the detective agency and Tuppence announcing she is pregnant.
After Lillie (1978), this is the second time that James Warwick (Tommy) and Francesca Annis (Tuppence) have played a married couple.
After The Case of the Missing Lady (1950), this was the second English language adaptation of the Tommy and Tuppence stories. In that production, Tommy and Tuppence were played by Ronald Reagan and Cloris Leachman.
Albert mentions a movie called The Ringer. This may been meant to relate to the 1932 German movie "Der Hexer" (The Ringer) based on an Edgar Wallace novel.
In Agatha Christie's stories, Tommy and Tuppence are typically secret agents rather than detectives. The series is based on the story collection "Partners in Crime" (1929), where the duo pose as the new owners of a detective agency while waiting for an enemy agent known only as "No. 16" to contact them. They solve the agency's cases because they enjoy the challenge.