The Secret Adversary is the second published detective fiction novel by British writer Agatha Christie, first published in January 1922.

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 Great War is over, and jobs are scarce. Childhood friends Tommy Beresford and Prudence "Tuppence" Cowley meet and agree to start their own business as The Young Adventurers. 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.

Reviews were generally positive on this adventure, which manages to keep the identity of the arch-criminal secret to the very end.

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. ...(Wikipedia.org)