Red Joan (2018)

English-born Joan Stanley (Dame Judi Dench), a Soviet and Communist Party sympathizer, becomes employed as a British government civil servant, and gets recruited by the K. G. B. in the mid 1930s.

She successfully transfers nuclear bomb secrets to Soviet Russia, which enables them to keep up with the west in the development of atomic weapons, and remains undetected as a spy for over a half a century.