The world's first programmable electronic digital computer was built by the British in 1943, helped shorten World War II, and was then deliberately destroyed and erased from history for 30 years. This episode tells the story of Colossus, the machine that cracked Hitler's high-command Lorenz cipher, and the unsung heroes who built it while America's ENIAC took the credit.

We follow the codebreaking journey from a German operator's fatal repeat-message blunder, through Bill Tutte reverse-engineering an unseen 12-wheel machine from pure statistics, to telephone engineer Tommy Flowers betting his own money on 1,500 vacuum tubes when the establishment said it was impossible. We also meet the Wrens who physically programmed the machine and rode the paper-tape beast at 30 miles per hour.

• How the XOR Vernam cipher worked and why Lorenz s ...