Childhood friends Tommy Beresford and Prudence Cowley (known as Tuppence) meet in London at the end of the First World War. Short of money and with no prospects, they decide to become adventurers and are soon working for the British Government. Their assignment is to find the missing Jane Finn who is suspected of having a secret treaty signed before the war, now missing. The treaty could cause a lot of trouble for the present government, and the mysterious 'Mr Brown' is somewhere in the background, plotting to bring down the government.

Kidnapping, treason and Bolsheviks await the adventurers before they solve the case.