Summertime, 1935.

On a lake near Berlin, a young man is out sailing when he glimpses a woman reclining in the prow of a passing boat. Their eyes meet - and one of history's greatest conspiracies is born. Harro Schulze-Boysen had already shed blood in the fight against Nazism by the time he and Libertas Haas-Heye began their whirlwind romance.

She joined the cause, and soon the two lovers were leading a network of antifascists that stretched across Berlin's bohemian underworld. Harro himself infiltrated German intelligence and began funnelling Nazi battle plans to the Allies, including the details of Hitler's surprise attack on the Soviet Union. But nothing could prepare Harro and Libertas for the betrayals they would suffer in this war of secrets - a struggle in which friend could be indistinguishable from foe.

Drawing on unpublished diaries, letters and Gestapo files, Norman Ohler spins an unforgettable tale of love, heroism and sacrifice.