In the Prologue, a man quietly gives important papers to a young American woman, as she is more likely to survive the sinking RMS Lusitania in May 1915.In 1919 London, demobilised soldier Tommy Beresford meets war volunteer Prudence "Tuppence" Cowley. They are both out of work and money. They form "The Young Adventurers, Ltd". Mr Whittington follows Tuppence to offer her work. She uses the alias "Jane Finn", which shocks Whittington. He gives her £50 and then disappears. Curious, they advertise for information regarding Jane Finn.The advertisement yields two replies. The first is from Mr Carter, whom Tommy recognises as a British intelligence leader from his war service; he tells them of Jane Finn aboard the Lusitania when it sank. She received a secret treaty to deliver to the American embassy in London. She survived but no trace has since been found of her or the treaty, the publication of which now would compromise the British government. They agree to work for him, despite his warnings of the dangerous Mr Brown. The second reply is from Julius Hersheimmer, an American multimillionaire and first cousin of Jane Finn, staying at the Ritz Hotel. Intent on finding her, he has already contacted Scotland Yard; Inspector Brown took his only photo of Jane, before a real inspector contacted him. They join forces with Julius, too.Whittington mentions the name Rita to Tuppence. Tommy and Tuppence find her among the survivors of Lusitania, Mrs Marguerite Vandemeyer. Whittington and Boris Ivanovitch leave Rita's flat as they reach the building. Tommy and Julius follow them; Tommy follows Boris to a house in Soho, while Julius trails Whittington to Bournemouth. Tommy eavesdrops on a meeting of Bolshevist conspirators, where he is caught. He delays his execution by claiming knowledge of the missing treaty. Tuppence secures the co-operation of Albert, the lift boy at Mrs Vandemeyer's residence, to obtain a job as her maid and hears Rita mention Mr Brown. The next visitor is Sir James Peel Edgerton, K C. On her afternoon off, Tuppence meets Julius at the Ritz. Julius had followed Whittington to a private clinic and seen him meet with a nurse; they both left before Julius could act. Tommy has not returned. Tuppence tells Mr Carter. Tuppence persuades Julius to seek advice from Sir James. Tuppence returns to Rita's flat early, interrupting Rita's preparations to flee. After a tussle, Tuppence forces Rita to admit she knows who Mr Brown is. Julius and Sir James arrive, Mrs Vandemeyer screams, collapses, and murmurs "Mr Brown" to Tuppence just before dying. Julius finds something in Rita's safe.