When Carruthers receives a letter from his friend Davies suggesting a Baltic sailing trip, the vision of a manned yacht quickly works its charm. The reality couldn't be more different. If the boat is underwhelming, Davies's ambition for their sailing trip is anything but. There has been suspicious German activity along the coast. The Medusa has already tried to destroy him. Why should anyone want to kill Davies? What are the Germans trying to hide? Nothing less than a plot to invade Britain. And only these two courageous Englishmen can stop them.