The year is 1901. Two ships sail off the north coast of Germany. One of the ships, the 'Dulcibella' , is a weather-beaten ex-lifeboat owned by a young Englishman, Arthur Davies, on a sailing holiday. The other is an opulent yacht owned by an apparently wealthy German, Dollmann. Davies becomes increasingly suspicious of Dollmann's behaviour. Could he be a spy? Wanting answers, Arthur contacts a friend in the Foreign Office who joins him aboard, only for them to be threatened by Dollmann and told to leave the area. Instead the two become even more involved and discover a German plot to invade Britain. They must get word urgently to British intelligence, but there are many forces who wish to keep them silent.