She tells them where she hid the treaty, but they find instead a message from Mr Brown. Sir James discovers Jane Finn, who has recovered her memory after an accident. Tommy returns to the Ritz and finds Tuppence. A young French woman at the house, Annette, arranges his escape, but refuses to leave herself. Meanwhile, Tommy follows Boris Ivanovitch, another of Rita's associates, to a house in Soho, where Tommy is taken prisoner while attempting to eavesdrop on a meeting of Bolshevist conspirators. Tuppence receives a telegram signed by Tommy and rushes after him. Mrs Vandemeyer screams, collapses, and murmurs 'Mr Brown' to Tuppence just before dying. Tuppence hears Mrs Vandemeyer mention Mr Brown and forces her to admit she knows his real identity. Tuppence obtains a job as Mrs Vandemeyer's maid and enlists the help of a young boy working there named Albert. Tommy and Tuppence's investigation leads them to the home of Mrs Marguerite 'Rita' Vandemeyer, a woman with several powerful friends, including Whittington and Sir James Peel Edgerton, K C. Tommy and Tuppence agree to find her, but Carter warns them about an enemy agent known only as 'Mr Brown.' Also searching for Jane Finn is her cousin Julius Hersheimmer, an American multimillionaire. Carter, an old friend of Tommy's from British intelligence, tells them that Jane Finn was a British agent who disappeared while attempting to deliver a secret treaty to the American embassy in London. Remembering a seemingly unrelated anecdote of Tommy’s, when Whittington asks Tuppence her name, she gives him the alias 'Jane Finn.' A shocked Whittington offers her £50 hush money and disappears. Their first client, a Mr Whittington, makes Tuppence a suspiciously generous offer. Out of work and money, they form 'The Young Adventurers, Ltd'. In 1919 London, demobilised soldier Tommy Beresford reunites with his childhood friend and war volunteer Prudence 'Tuppence' Cowley. Reviews were generally positive on this adventure, which manages to keep the identity of the arch-criminal secret to the very end. They are hired for a job that leads them both to many dangerous situations, meeting allies as well, including an American millionaire in search of his cousin.

Childhood friends Tommy Beresford and Prudence 'Tuppence' Cowley meet and agree to start their own business as The Young Adventurers. The Great War is over, and jobs are scarce. 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. The UK edition retailed at seven shillings and sixpence (7/6) and the US edition at $1.75. The Secret Adversary is the second published detective fiction novel by British writer Agatha Christie, first published in January 1922 in the United Kingdom by The Bodley Head and in the United States by Dodd, Mead and Company later in that same year.

The secret adversary book