This documentary is about cracking the German High Command's Lorenz machine cypher or "Tunny" as the Brits called it. Unlike Enigma, which was invented in the 1920s and sold commercially (yes you could buy an Enigma machine before WW II and several were in use in England), Lorenz was state-of-the-art. It operated on telex transmissions rather than morse radio and it's operation was seamless; the sender typed in the message, their Lorenz machine encrypted it and the receiver's Lorenz machine decrypted it, all without any human intervention. The Allies had never seen a Lorenz machine and had to infer how it worked.
This remarkable feat was done by a young mathematician called Bill Tutte. Once the logical structure of the Lorenz machine was deduced Turing and Max Newman set about cracking the cypher. Newman designed an electro-mechanical machine nicknamed "Heath Robinson" to automate the laborious process known as Turingery. Like the cartoonists fabulously complex contraptions Robinson was prone to failure. A young electrical engineer called Tommy Flowers who worked at the Post Office Research Station designed an electronic replacement called Colossus - this was the worlds first electronic computer. By the end of the war there were 10 Colossus computers at Bletchley Park and the Allies were routinely reading all of the German High Commands telex traffic - Hitler's direct commands to his generals!
So if you've never heard of Bill Tutte, the Lorenz cypher, Tunny, Max Newman, Tommy Flowers or Colossus this excellent documentary will open a whole new chapter in WW II code-breaking for you. The documentary is available to watch on the BBC's website if you live in the UK. It's also available to download (illegally) from the usual bit-torrent search engines.
A working replica of Colossus at Bletchley Park