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Davies reveals he was nearly wrecked after 'Herr Dollmann' deliberately lured him into a shallow channel in a gale; Dollmann turns out to be a renegade former British naval officer working for Germany. By sounding channels and tracking tides, the pair discover a plan to move troops in tug-towed barges through the tidal shallows and land them on England's undefended east coast.
The book argues that Britain's naval thinking was fixed on blue-water fleet battles and on France, which left the North Sea coast facing Germany without a naval base, fortifications, or a reserve trained for coastal waters. An invasion could exploit this blind spot because nobody was watching the shallow approaches.
Davies can read the plan only because he has spent months sounding the channels, timing tides, and learning which sands dry at low water. The novel treats knowledge of hydrography as the real tradecraft: a patient amateur with local, physical knowledge sees an operational possibility that official intelligence misses because it never looked at the ground itself. Whoever has mapped the terrain in detail can tell what a given piece of that terrain is useful for.
On Memmert, a salvage company works openly at raising the gold of a sunken frigate. The operation gives cover for boats, tugs, and men to move through the channels and study them, and it explains the traffic to any observer.