Just after World War I, Tommy Beresford and Tuppence Cowley are desperately short of money. With a shortage of job opportunities, they form a partnership, hiring themselves out as "young adventurers, willing to do anything, go anywhere." In their first dangerous assignment, they must use all their ingenuity to save not only their own lives but also the life of a mysterious girl.

The girl in the photograph has been missing for five years. Neither her body nor the secret documents she was carrying have ever been found. Now postwar England's economic recovery depends on finding her and getting the papers back. But the two young Brits working undercover for the ministry, Tommy and Tuppence, know only that her name was Jane Finn and the only photo of her is in the hands of her rich American cousin. They don't yet know about a mysterious and ruthless man called "Mr. Brown" or the beautiful but sinister older woman who knows all about Jane Finn—and therefore must die.