Born and raised in America, Mildred Harnack was twenty-six when she enrolled in a PhD programme in Germany 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 recruited Germans into the resistance, helped Jews escape, plotted acts of sabotage and collaborated in writing leaflets that denounced Hitler and called for revolution.

Her co-conspirators circulated through Berlin under the cover of night, slipping the leaflets into mailboxes, public restrooms, phone booths. When the first shots of the Second World War were fired 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.

During a hastily convened trial at the Reichskriegsgericht - the Reich Court-Martial - a panel of five judges sentenced her 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. Harnack's great-great-niece Rebecca Donner draws on extensive archival research and newly discovered documents in her family archives in this astonishing work of nonfiction.

Fusing elements of biography, political thriller and scholarly detective story, Donner brilliantly interweaves letters, diary entries, notes smuggled out of a Berlin prison, survivors' testimony, and a trove of declassified intelligence documents into a powerful, epic story, reconstructing the moral courage of an enigmatic woman nearly erased by history.