The Secret Adversary, Hardcover

In the Prologue, a man aboard the RMS Lusitania on 7 May 1915 quietly gives important papers to a young American woman, as she is more likely to survive the sinking ship.
In 1919 London, demobilised soldier Tommy Beresford meets war volunteer Prudence "Tuppence" Cowley, both out of work and money. They form "The Young Adventurers, Ltd," planning to hire themselves out with "no unreasonable offer refused." They are overheard by Mr Whittington who follows Tuppence to offer her a position. He is shocked when she gives her name as "Jane Finn." Whittington sends her away with some money, then disappears without a trace. They advertise for information regarding Jane Finn.
The advertisement yields two immediate replies. They first meet with Mr Carter, whom Tommy recognises as a leader in British intelligence from his war service. Mr Carter tells of Jane Finn aboard the RMS Lusitania when it sank in 1915. She received a secret treaty to deliver to the American embassy in London. She survived, but the government has found neither Jane Finn nor the draft treaty since. Publication of the treaty now would compromise the British government. Tommy and Tuppence agree to work for Carter. He warns them of the elusive and merciless figure known as Mr Brown. Next they meet with Julius Hersheimmer, an American multimillionaire who is the first cousin of Jane Finn, staying at the Ritz Hotel. He is intent on finding her. He has already contacted Scotland Yard; an Inspector named Brown took his only photo of Jane, before a real inspector contacted him. Tommy and Tuppence join forces with Julius.