This, the earliest of Hammond Innes's sea stories is a fairly typical WWII adventure story, with a pair of fairly ordinary Englishmen thwarting the nefarious designs of Nazi spies. It was written, however, during the very earliest weeks of the war and is extraordinarily prescient; the threat to shipping from submarine warfare was known, but U-boat pens did not exist until some years later.Innes also scores extra points (from me) for including a plucky and intrepid female journalist investigate the mystery and for the Cornish setting. I would point out that Cornish wreckers only seem to have ever existed in fiction, but can't complain of him using the idea.