In 1941, the small counter-intelligence section known as RU40, run by Clarence Wilkinson, was instrumental in shutting down a German counterfeiting operation and neutralising its members. Many in the intelligence service considered the case had been satisfactorily concluded, although it was never discovered exactly how instructions were passed between the German Abwehr and its agents in Britain. Due to his dislike of loose ends, Wilkinson is determined to uncover the method employed and, together with Jenny Matheson and Tom Adams, commences a low-key investigation which takes them to the south coast of England.

Too late, it is realised that their uninvited presence and enquiries so close to Dover could jeopardise the recovery by a secret Dover unit of a heroic intelligence officer from enemy-occupied France. They are quickly drawn into the periphery of the recovery scheme, together with a young police constable, to avoid unintentionally disrupting the operation.

During the course of the investigation, Jenny Matheson becomes the object of attention of a romantic nature, which leads her to take extreme action as the only way to fend off the unwanted and increasingly persistent advances.
Meanwhile, in France, a senior German Abwehr officer is finalising his plan for an operation designed to cause major embarrassment to Britain, and inevitably, the activities of both protagonists converge.