Secret Adversary

The Secret Adversary is a work of detective fiction by Agatha Christie first published in the United Kingdom by The Bodley Head in January 1922 and in the United States by Dodd, Mead and Company later in that same year. The UK edition retailed at seven shillings and sixpence (7/6) and the US edition at $1.75. It is her second published novel. The book introduces the characters of Tommy and Tuppence who feature in three other Christie novels and one collection of short stories; the five Tommy and Tuppence books span Agatha Christie's writing career.

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 "The Young Adventurers, Ltd", planning to hire themselves out with "[n]o unreasonable offer refused." They are overheard by a 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 the story of Jane Finn, an American heading for Paris on 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 this is a dangerous mission, because of the elusive and merciless figure known as Mr Brown. Next they meet with Julius Hersheimmer, an American man who is the first cousin of Jane Finn and a multimillionaire, 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.