India, July 29 -- "Tommy, old thing!"

"Tuppence, old bean!"

A chance meeting on a London Tube platform sets the stage for one of Agatha Christie's most enduring detective partnerships. Thomas Beresford and Prudence 'Tuppence' Cowley return to each other's lives in The Secret Adversary (1922). World War I has just ended, jobs are scarce, and the future looks uncertain.

They go on to do what any sensible young people wouldn't: place an advertisement. "Two young adventurers for hire. Willing to do anything, go anywhere. Pay must be good. No unreasonable offer refused."

It's one of Christie's most irresistible beginnings. Before long, the pair are chasing missing government papers, evading spies, and stumbling into far more danger than th...