One of the earliest English spy novels, The Riddle of the Sands by Erskine Childers, is the basis for this movie. In the story, it is 1901, and England is at the height of its power and influence in the world. Arthur Davies (Simon MacCorkindale) is out sailing in his yacht in the North Sea. Off the coast of Germany, he has a set-to with a local sea-captain, Dollmann (Alan Badel). The encounter itself was odd enough, but adding to it's strangeness is Arthur's nagging sense that the German captain was known to him, somehow. He discovers that Dollman is actually a British naval officer who has gone bad and calls on his old college friend, Charles Carruthers (Michael York) to help him investigate this phenomenon. Eventually, the two men discover that the Kaiser Wilhelm has planned to secretly invade and conquer Britain, and they are the only ones who can prevent it.