Tommy Beresford and Prudence "Tuppence" Cowley are young, in love… and flat broke. Just after the Great War, there are few jobs available, and the couple is desperately short of money. Restless for excitement, they decide to embark on a daring business scheme: Young Adventurers Ltd.—willing to do anything, go anywhere.

Hiring themselves out proves to be a smart move for the couple. In their first assignment for the mysterious Mr. Whittington, all Tuppence has to do is take an all-expenses-paid trip to Paris and pose as an American named Jane Finn. But with the assignment comes a bribe to keep quiet, a threat to her life, and the disappearance of her new employer.

Now, their newest job is playing detective. Where is the real Jane Finn? The mere mention of her name produces a very strange reaction all over London. So strange, in fact, that they decide to find this mysterious missing lady. She has been missing for five years. Neither her body nor the secret documents she was carrying have ever been found.

Now, post-war England's economic recovery depends on finding her and getting the papers back. But the two young adventurers, working undercover for the British ministry, know only her name and that the only photo of her is in the hands of her rich American cousin. It isn’t long before they find themselves plunged into more danger than they ever could have imagined—a danger that could put an abrupt end to their business… and their lives.