This episode covers the early allied efforts at cracking the German Enigma system, the establishment of Station X at Bletchley Park and the gathering of the eclectic group of geniuses who were to break the "unbreakable" German Enigma encryption system. The programme gives insight into the inner-workings of the Enigma machine, and the manner in which Bletchley's brilliant boffins managed to outsmart it.

The programme discusses The Herivel Tip, an imaginative cracking method which relied on the laziness of the Enigma operators when setting up their machines in the morning. Also explored is the role of Station X in the air and sea battles of 1941 and the part played by Alan Turing in cracking Germany’s naval messages.

The programme starts with the 1941 Invasion of Russia. Contributors include David Kahn In a speech Winston Churchill publicises mass executions and the systematic slaughter of Jews, feeling the outrage greater than the imperative to protect the source. Loris Gherardi's role in the Theft of the Black Code, and its continued use under Colonel Norman Fiske and Bonner Fellers, is reconstructed. Also, Turing’s development of the Bombe, a machine used to automate the code-breaking process.