Born and raised in America, Mildred Harnack was twenty-six when she enrolled in a PhD programme in Berlin and witnessed the meteoric rise of the Nazi party. In 1932, she began holding secret meetings in her apartment – a small band of political activists that by 1940 had grown into the largest underground resistance group in Berlin.

She helped Jews escape, plotted acts of sabotage and wrote leaflets denouncing Hitler’s regime. On the outbreak of the Second World War she became a spy, couriering top-secret intelligence to the Allies. On the eve of her escape to Sweden, she was ambushed by the Gestapo. At a Nazi military court she was sentenced to six years at a prison camp, but Hitler overruled the decision and ordered her execution. On 16 February 1943, she was strapped to a guillotine and beheaded.

Fusing elements of biography, political thriller and scholarly detective story, Harnack’s great-great-niece Rebecca Donner brilliantly interweaves family archives, original research, exclusive interviews with survivors, and a trove of declassified intelligence documents into a powerful, enthralling story, reconstructing the moral courage and previously untold story of an enigmatic woman nearly erased by history.