The Secret Adversary By Agatha Christie : A Hercule Poirot Mystery

A Mr Whittington overhears their conversation and offers Tuppence their first assignment, but when she tells him her name is ‘Jane Finn’ he acts very strangely and thinks she is blackmailing him. From then on Tommy and Tuppence set out to find Jane Finn, a name Tommy had overheard from a conversation in the street.