The Secret Adversary

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