Summoned to attend a patient in a house near the Romney Marsh, Rawlings does not suspect that he is walking into a web of conspiracy, intrigue and mystery. Until he discovers a body near a deserted church, bearing a coded document.

Rawlings reports the case to London's famous blind magistrate John Fielding who identifies the victim as a French spy master. So Rawlings returns to the marshes to investigate who, among the colourful local characters, could be harbouring politically explosive secrets.