In 1939, several hundred people - students, professors, international chess players, junior military officers, actresses and debutantes - reported to a Victorian mansion in Buckinghamshire: Bletchley Park. This was to be 'Station X', the Allies' top-secret centre for deciphering enemy codes.

Their task was to break the ingenious Enigma code used for German high-level communications. The settings for the Enigma machine changed continually and each day the German operators had 159 million million million different possibilities. Yet against all the odds this gifted group achieved the impossible, coping with even greater difficulties to break Shark, the U-Boat Enigma, and Fish, the cypher system used by Hitler to talk to his guards.