Childhood friends Tommy Beresford and Prudence Cowley, affectionately 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 been in possession a secret treaty signed before the war and which is now missing. Although the treaty is no longer valid, it can cause lots of trouble for the present government. Kidnapping, treachery and Bolsheviks await our adventures before they solve the first of their many cases.