In 1941, Charlotte Pommer graduated from medical school at the University of Berlin and went to work for Hermann Stieve, head of the school’s Institute of Anatomy. The daughter of a bookseller, Pommer had grown up in Germany’s capital city as Hitler rose to power. But she didn’t appreciate what the Nazis meant for her chosen field until Dec. 22, 1942. What she saw in Stieve’s laboratory that day changed the course of her life—and led her to a singular act of protest.

Stieve got his “material,” as he called the bodies he used for research, from nearby Plötzensee Prison, where the courts sent defendants for execution after sentencing them to die. In the years following the war, Stieve would claim that he dissected the corpses of only “dangerous criminals.” But on that day, Pommer saw in his laboratory the bodies of political dissidents. She recognized these people. She knew them.

On one table lay Libertas Schulze-Boysen, granddaughter of a Prussian prince. She’d been raised in the family castle, gone to finishing school in Switzerland, and worked as the Berlin press officer for the Hollywood studio Metro-Goldwyn-Mayer. She joined the Nazi Party in 1933. On a hunting party, she flirted with Hermann Göring, commander of the Luftwaffe, the German air force. But in 1937 Schulze-Boysen joined the resistance with her husband, Harro, a Luftwaffe lieutenant. They helped form a small rebel group the Nazis called the Red Orchestra. When Libertas started working for Hitler’s movie empire in 1941, she gathered photos of atrocities from the front for a secret archive. Harro was transferred to Göring’s command center and with other dissidents started passing to the Soviets detailed information about Hitler’s plan to invade Russia. The Gestapo decoded their radio messages in 1942 and arrested Harro at the end of August. They came for Libertas eight days later. Both she and her husband were sentenced to death for espionage and treason....