The narrative opens with an unexpected invitation for a yachting trip in the Baltic, which draws an ambitious young Foreign Office clerk into a strange maritime quest. His companion, an earnest and experienced sailor, is convinced that a fellow Englishman living abroad is engaged in treasonous activities involving the German navy. Their initial skepticism and discomfort give way to a rigorous examination of treacherous sandbanks and tidal channels behind the Frisian Islands. Along the way, they encounter mysterious locals, naval officers, and subtle hints of surveillance that confirm their darkest suspicions.
The protagonist struggles with the personal complications of his friend's past attachments while they navigate both literal tempests and diplomatic intrigue. As they press westward toward their ultimate destination, they realize their investigation touches upon vital secrets of naval defense and coastwise navigation. The tension escalates as they must reconcile their patriotic duty with the complex web of deceit spun by their adversary.
