On August 4, 1944, two months after D-Day, the Allied codebreaking operation at Bletchley Park was running at full capacity — and over half its workforce was women. Among them was Joan Clarke, the only woman ever to reach the senior cryptanalyst grade at Bletchley. She sat in Hut 8 next to Alan Turing. She had helped break Enigma.

Clarke was born on June 24, 1917, in West Norwood, London. She won a scholarship to Girton College, Cambridge, achieved a double first in mathematics — the highest classification possible — but was awarded only a "Titular" degree because Cambridge did not grant full degrees to women until 1948. She was recruited into Bletchley in 1940 by Gordon Welchman, who had noticed her work.

At Hut 8, she worked on naval Enigma, the version of the cipher used by German submarines in the Atlantic. Breaking it was essential to keeping Allied supply lines open. Turing was leading the team; Clarke was doing the cryptanalysis. He regarded her as the best codebreaker in the hut. She was promoted to deputy head of Hut 8 and given the title "linguist" to justify paying her at the male rate — because there was no official grade for a woman doing what she was actually doing.

Turing proposed marriage to her in 1941. He told her shortly afterward that he was homosexual. They remained close friends and colleagues for the rest of his life.

After the war, all Bletchley personnel were sworn to secrecy under the Official Secrets Act. Most of them kept the secret until they died. Clarke kept it for thirty years — until the Ultra secret was officially declassified in 1974. She was fifty-seven.

She spent her post-war career in GCHQ, the British intelligence agency that succeeded Bletchley. She worked there until 1977, publishing research in numismatics in her spare time. She died in 1996. Her role in the Enigma project was not publicly documented until after her death, when the Bletchley records were released.

The 2014 film The Imitation Game cast her as a supporting character whose primary narrative function was to reveal Turing's sexuality. She received one line of acknowledgment for her mathematical contribution.