Summoned to attend a patient in a house near the lonely 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.

Back in London, Rawlings and the Blind Beak identify the victim as a French spy master, come to awaken two sleeping spies, described only as The Frog and The Moth. John returns to the marshes to investigate the murder - and to unmask the secret agents.

A motley bunch of characters emerges: from Sir Ambrose Ffoulkes, a hearty local baronet, his long-suffering wife, the Rector of Winchelsea and his two beautiful dauighters to the enigmatic Marquis of Rye; from the unlikely head of the British secret service to the French spy dwelling in London under an assumed identity. But do any of them also go by the names of The Frog and The Moth?