Historical BBC documentary that depicts the work of codebreaker Gordon Welchman. During the Second World War, Welchman worked alongside Alan Turing at Bletchley Park, breaking German ciphers, and played a key role in cracking the Enigma machine. Following the war he moved to the United States and, after retiring, released a book detailing his work for both the US and UK military which led to him being ostracised by the National Security Agency.