Through a gallant attempt, the convoy is saved, but the doughty little destroyer is outgunned and sunk. Only a few sailors survive, including Lieutenant John Braithwaite. Wounded and unfit for sea duty, Naval Intelligence Division (NID) recruits him for field work based on his knowledge of East Africa and fluency in German. The mission - discover the secret of the mysterious enemy light cruiser SMS Aachen operating out of German East Africa, which is devastating British shipping. How is Aachen always able to find targets? How does she always evade the Royal Navy?

Arriving in Zanzibar, he meets the beautiful and charming Alice, code name MINSTREL, a widow serving as NID's resident agent. Going aboard the enemy cruiser disguised as a native laborer, Braithwaite discovers references to "The Machine" and charts marked with the term "Station Wilhelm" located in the heart of Africa on Lake Nyasa.

Together, Braithwaite and MINSTREL travel up the Zambezi River in search of the Aachen's secret, but they must battle the elements as well as the Germans as they go deep into the Dark Continent.