It is January 1721. London is reeling from the effects of the greatest financial scandal of the age, the collapse of the South Sea Bubble. William Spandrel, a penniless mapmaker, is offered a discharge of his debts by his principal creditor, Sir Theodore Janssen, a director of the South Sea Company, on one condition: he must secretly convey an important package to a friend of Janssen's, Ysbrand de Vries, in Amsterdam.

The package safely delivered, Spandrel barely survives an attempt on his life, only to be blamed for the murder of de Vries himself. When de Vries's secretary, his English wife and the package itself go missing shortly afterwards, Spandrel realizes that he has become a pawn in several people's games. British Government agents, and others, are on his trail, believing that the mysterious package contains secrets that could spark a revolution in England.

Spandrel's only chance of survival is to recover the package and place its contents in the right hands. But whose are the right hands? And what exactly are the contents?