Mr Brown is after a treaty which was drafted during the great war but got lost, this treaty at no cost should fall in Mr Brown's hands because if it does then it will ensure a revolution which might bring down the British Government. Tommy and Tuppence are now hired by the government as spies, who want them to find out as much as they can about this enigmatic business. The only problem is that the mysterious Mr Brown is watching their every step, more closely than they can ever imagine.
The dedication of the book reads:
"To all those who lead monotonous lives in the hope that they experience at second hand the delights and dangers of adventure"