The Secret Adversary (1922) – Agatha Christie | First Edition Identification Guide

In 1919 London, demobilised soldier Tommy Beresford reunites with his childhood friend and war volunteer Prudence “Tuppence” Cowley. Out of work and money, they form “The Young Adventurers, Ltd”. Their first client, a Mr Whittington, makes Tuppence a suspiciously generous offer. 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. 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. 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.
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. Tuppence obtains a job as Mrs Vandemeyer’s maid and enlists the help of a young boy working there named Albert. Tuppence hears Mrs Vandemeyer mention Mr Brown and forces her to admit she knows his real identity. Mrs Vandemeyer screams, collapses, and murmurs “Mr Brown” to Tuppence just before dying. Tuppence receives a telegram signed by Tommy and rushes after him.
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 Tuppence.
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.