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 Although the treaty is no longer valid, it can cause lots of trouble for the present government.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.

The Beresfords first real case involves the theft of a valuable pink pearl. Among the guests staying at the Kingston-Bruce's are Mr. & Mrs. Hamilton Betts and Lady Laura Barton. While Colonel Kingston-Bruce is trying to interest Mr. Betts in investing in his company, Mrs. Betts valuable pink pearl has been stolen. Not wanting any adverse publicity, Kingston-Bruce calls in Blunt's International Detective Agency. Colonel Kingston-Bruce feels sure that the thief is daughter Beatrice's inappropriate friend Mr. Rennie, while Mr. Rennie and Beatrice suspect each other. It is also probable that Lady Laura may be a kleptomanic and have taken the pearl. Leaving Tuppence completely in the dark, Tommy rushes off the Scotland Yard with a photo and finger prints of the real thief.

Murder is in the air when Captain Sessle is found dead on a golf course. Tommy is convinced the police have arrested the wrong man, and he and Tuppence set out to find the real killer.