Tommy Beresford, an unimaginative but charming young man, encounters a childhood friend, Tuppence Cowley, after he is released from service in WWI. Jobless, they decide to start Young Adventurers Ltd. promising potential clients: "Willing to do anything. Go anywhere.... No unreasonable offer refused." The Jazz Age duo marry at the end of their first adventure, The Secret Adversary (1922).

In the short story collection, Partners in Crime, Tommy and Tuppence take over a nearly-bankrupt detective agency and solve cases in the manner of assorted great detectives including Sherlock Holmes, Father Brown-- and Hercule Poirot. The couple eventually have a set of twins, Deborah and Derek Beresford, and adopt a daughter, Betty Beresford.

The Beresfords age gracefully in the course of three subsequent books: N Or M?, By The Pricking of My Thumbs and Postern of Fate, where they are seen as retired grandparents who still love a good mystery. To the end, Tommy remains the slower thinker whose natural caution serves as the perfect foil to Tuppence's impetuous nature.