In 1920 London, demobilised soldier Tommy Beresford reunites with his childhood friend and war volunteer Prudence "Tuppence" Cowley. In passing, he mentions that he overheard a man mention the name "Jane Finn," which he finds ridiculous. Out of work and money, they form "The Young Adventurers, Ltd". Their first client, a Mr Whittington, makes Tuppence a suspiciously generous offer. Remembering Tommy's anecdote, when Whittington asks Tuppence her name, she gives him the alias "Jane Finn." A shocked Whittington offers her £50
hush money and asks her to return the following day, but when she returns, the office has been suddenly closed. They decide to advertise for information about "Jane Finn," and receive two replies, from a Mr. Carter and from Mr. Julius Hersheimmer. Carter, who Tommy recognizes from his wartime service in British intelligence, tells them that Jane Finn is believed to have received a secret treaty from a British agent during the sinking of
RMS Lusitania in May 1915, as she was
more likely to survive. According to Mr. Carter, if the terms of the treaty were disclosed, it would cause the government to fall and precipitate a general strike. Mr. Carter hires Tommy and Tuppence to find her, but Carter warns them about an enemy agent known only as "Mr Brown." They then get in touch with Julius Hersheimmer, who turns out to be an American multimillionaire, and Jane Finn's cousin, who has come to London to find her. 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. Under cover, Tuppence obtains a job as Mrs. Vandemeyer's maid and enlists the help of a young boy working there named Albert. Edgerton cryptically suggests that Tuppence might be better off working for someone else. This leads Tommy and Tuppence to go to Edgerton's office to ask about the remark. When Tuppence resumes her disguise and returns to Vandemeyer's house, Vandemeyer discovers her deception, and holds her at gunpoint, until Tuppence wrests the gun away. Tuppence offers Vandemeyer a large bribe to tell the whereabouts of Jane Finn, but when Hersheimmer and Edgerton arrive, she screams and faints. They leave her in her bed but lock her in her room because of their fear of Mr. Brown. When they return in the morning, Vandemeyer is dead. 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. A young French woman at the house, Annette, arranges his escape, but refuses to leave herself. Tommy returns to the Ritz and finds that Tuppence has just left in a hurry. Tommy and Hersheimmer find the telegram that caused Tuppence to flee, and attempt to follow her. They find that the address she was directed to is a long abandoned house. After a week of questioning the locals and searching the area they give up. Sir James discovers Jane Finn, who has recovered her memory after an accident. She tells them where she hid the treaty, but they find instead a message from Mr. Brown. While searching for writing paper in Julius's drawer, Tommy finds a photograph of Annette. Tommy concludes that Annette is the real Jane Finn and the Jane Finn they met was a plant to stop their investigation. He gets an original copy of the telegram sent to Tuppence and sees that her destination was altered on the copy he read. Tommy and Albert proceed to the correct destination. Hersheimmer arranges for the release of Tuppence and Annette. At Sir James's residence, Jane tells her story: after receiving the packet, she became suspicious of Mrs Vandemeyer. Jane placed blank sheets in the original packet, sealing the treaty inside magazine pages. Travelling from Ireland, she was mugged and taken to the house in Soho. Perceiving the intent of her captors, Jane faked amnesia, conversing only in French. She hid the treaty in a picture frame in her room and has maintained her role as "Annette" ever since. Tuppence suspects that Hersheimmer is Mr. Brown. Sir James agrees, adding that the real Hersheimmer was killed in America and that his imposter killed Mrs. Vandemeyer. They rush to Soho, recovering the treaty at the house. Sir James identifies himself as the true Mr. Brown, and announces his plan to kill them, wound himself, and then blame it on the elusive Mr. Brown. Julius and Tommy, who are hiding in the room, overwhelm Sir James. He commits suicide using poison concealed in his ring, the compelling evidence to persuade Mr. Carter of his old friend's guilt. Tommy’s role in solving the mystery persuades his estranged rich uncle to support him financially and make Tommy his heir. The novel ends with both Hersheimmer and Jane, and Tommy and Tuppence, engaged to marry. ==Characters==