The war in France is a stalemate. The "Mill," a brutal, industrial-scale meat grinder, is chewing up British, American, and German soldiers by the thousands. On the front lines, Sergeant Frank Miller and Corporal Thomas Atkins, veterans of the battle for Britain, fight for every bloody inch of mud, their survival, and the fate of the war.

Elsewhere, there is the "shadow war" being fought. Raymond Clinton, the unassuming clerk who became a national hero after hunting down the spy who orchestrated the invasion of Britain, has been pulled from the light. He is recruited by the mysterious Mr Smith into a new, clandestine intelligence service.

His first assignment: Berlin. He must infiltrate the heart of Britain's key ally, a city strained by a war on two fronts. His mission is to assist a secret cell of German counter-intelligence, led by the aristocratic Elsa von Lützow, in hunting a deadly Russian spy ring. This "Viper's Nest" is run by Katerina Volkov—a brilliant, ruthless fanatic operating in the shadow of her infamous handler and with a personal vendetta. Her goal: to steal the plans for the Allies' final, war-winning offensive.