When Charles Carruthers receives a letter from a university friend Arthur Davies asking if he wants to join him yachting in the Frisian Islands off the northern coast of Germany, he envisions a pleasant holiday from his job in the Foreign Office. Upon arriving, he finds that the 'yacht' is a converted life boat with no other crew other than the two of them, and that his friend's plan is to chart the channels and islands of the shore against the possibility of war with Germany. As the season advances and the weather worsens, it becomes clear that there is a deeper secret hidden in the off shore islands, and forces that will do anything to stop Carruthers and Davies from discovering the answer to The Riddle of the Sands!