Just after World War One, Tommy Beresford and Tuppence Cowley are desperately short of money. With few jobs available, they decide to form a partnership, hiring themselves out as "young adventurers, willing to do anything, go anywhere." In their first assignment, Tommy and Tuppence must use all of their ingenuity to save not only their own lives, but the life of a mystery woman named Jane. Where is Jane Finn? The mere mention of her name produces a very strange reaction all over London.