Six years have passed since the Beresfords began their sleuthing partnership in The Secret Adversary. Tommy now has a desk job with the British Secret Service, and Tuppence, much to her displeasure is at home, though when the Chief of British Intelligence asks them to take over the International Detective Agency, both jump at the chance of new adventures.
The fifteen stories contain parodies of fictional detectives who were well-known to readers of the 1920s. In each story Tommy and Tuppence assume the mannerisms and methods of a different detective or detective team, including Sherlock Holmes.
The stories were published together in 1929. The entire book was adapted in 1953 starring Sir Richard Attenborough and his wife Sheila Sim as Tommy and Tuppence. Ten of the stories were adapted by LWT in 1984 and starred James Warwick and Francesca Annis.
The Beresfords' old friend, Mr. Carter (who works for an unnamed government intelligence agency) arrives bearing a proposition for the adventurous duo. They are to take over 'The International Detective Agency', a recently cleaned out spy stronghold, and pose as the owners so as to intercept any enemy messages coming through. But until such a message arrives, Tommy and Tuppence are to do with the detective agency as they please - an opportunity that delights the young couple. They employ the hapless but well-meaning Albert, a young man also introduced in The Secret Adversary, as their assistant at the agency.
Eager and willing, the two set out to tackle several cases. In each case mimicking the style of a famous fictional detective of the period, including Sherlock Holmes and Christie's own Hercule Poirot.
At the end of the book, Tuppence reveals that she is pregnant, and as a result will play a diminished role in the spy business.
The stories and their detective parodies
- A Fairy in the Flat / A Pot of Tea - Introduces the setup of Tommy and Tuppence at The International Detective Agency. Reminiscent of Malcolm Sage, detective (1921) by Herbert George Jenkins.
- The Affair of the Pink Pearl - This first case is in the vein of the detective Dr Thorndyke by R. Austin Freeman.
- The Adventure of the Sinister Stranger - An espionage story, following in the footsteps of Valentine Williams and the detective brothers Francis and Desmond Okewood. One of the Williams' books in particular - The Man with the Clubfoot (1918) is named by Tuppence in the story.
- Finessing the King / The Gentleman Dressed in Newspaper - This two part story is a spoof of the nowadays almost forgotten Isabel Ostrander, with parallels to the story The Clue in the Air (1917) and the detectives Tommy McCarty (an ex-policeman) and Dennis Riordan (a fireman).
- The Case of the Missing Lady - This story references Conan Doyle's Sherlock Holmes story The Disappearance of Lady Frances Carfax (1911).
- Blindman's Buff - Matches Clinton H. Stagg's stories around the blind detective Thornley Colton.
- The Man in the Mist - In the style of G. K. Chesterton's Father Brown stories.
- The Crackler - A spoof on Edgar Wallace's style of plotting.
- The Sunningdale Mystery - The tale is in the style of Baroness Orczy's The Old Man in the Corner (1909) with Tuppence playing the role of journalist Polly Burton and Tommy tying knots in a piece of string in the same way as Orczy's character, Bill Owen.
- The House of Lurking Death - Recreates the style of A. E. W. Mason and his French detective Inspector Hanaud.
- The Unbreakable Alibi - Modelled after Freeman Wills Crofts, known for his detective stories centred around alibis and the Scotland Yard detective Inspector Joseph French.
- The Clergyman's Daughter / The Red House - A two part story, this is a parody on detective Roger Sherringham by Anthony Berkeley, with plot elements reminding of The Violet Farm by H. C. Bailey (although the latter was not published until 1928).
- The Ambassador's Boots - Following the stype of H. C. Bailey with Dr. Reginald Fortune and Superintendent Bell as the parodied detectives.
- The Man Who Was No. 16 - This story parodies Christie's own The Big Four, featuring Hercule Poirot.