Heroes of War: Poland S01E05 - Cracking Enigma: The Untold Story

Working for the Cypher Bureau of the Polish General Staff, and using only encrypted messages, mathematician Marian Rejewski and two colleagues reverse-engineered an Enigma machine and built a "cryptologic bomb" to translate messages written by it. Weeks before the German and Russian invasions of September 1939, the Poles passed this information to Britain and France, and it became the unacknowledged basis of code-breaking operations at Bletchley Park.