"Two young adventurers for hire. Willing to do anything, go anywhere. Pay must be good. No reasonable offer refused"

Tommy Beresford and Prudence "Tuppence" Cowley are broke. What are two young people in post–WWI London to do in such a situation except start up a new and exciting business: Young Adventurer's Ltd.

Almost as soon as they make the leap into self-employment a case falls into their lap. A mysterious man wishes to hire Tuppence for a job. All she has to do is impersonate a young American woman in Paris. Easy!

Except all is not as it seems, and soon the pair are hurled into the perilous world of espionage as they try to discover who the mysterious Jane Finn is, and where she has hidden secret documents that could take down the British government.