Davies contacts his friend, Carruthers. He proposes they take a boating trip in the Baltic Sea. Carruthers readily agrees. Both men are up for an adventure. And adventure is what they get, though of a kind more dangerous than what they had envisaged. The Englishmen discover there has been German activity along the coast. Davies has already tangled with the Medusa, captained by the sinister Dollman. It does not stop with Dollman. There are bigger games afoot. Davies and Carruthers learn that the plot is nothing less than a German invasion of Britain. Who can thwart this devilish plot? It seems the safety of the country is in the hands of two brave Englishmen.