A chance invitation to a yachting holiday in the Baltic leads two men to uncover a clandestine German military operation. Carruthers, a bored civil servant, joins his acquaintance Davies on a sailing expedition through the Frisian Islands. As they navigate the treacherous shoals and shifting tides of the German coast, they realize their presence is being monitored by hostile forces. The narrative unfolds through a series of log entries and letters, documenting their transition from amateur sailors to amateur intelligence agents operating under extreme physical duress.

Readers frequently highlight the technical precision of the sailing sequences as a defining feature of the narrative. Discussion often centers on the book's role as a precursor to the modern spy thriller genre, noting its influence on subsequent espionage literature. Critics often point to the atmospheric tension created by the desolate, shifting landscape of the Frisian coast. The pacing is noted for its methodical build, prioritizing the slow accumulation of evidence over rapid action sequences. Many readers appreciate the authentic, grounded approach to the protagonist's transformation from an ordinary man into an operative.