On 31st August 1939, German SS Troops in Polish uniforms launched a false attack on a German radio station on the border. The following day, Hitler used this as his pretext for invading Poland with two enormous pincer movements. He believed Britain and France wouldn't react but he was mistaken and both declared war on Germany on 3rd September.

Appeasement had given way to abject betrayal and the whole of Europe was plunged into war once more. The Germans believed they had a secret weapon that would give them a crucial advantage over the Allies: Enigma. This cipher machine allowed the German High Command to exchange coded messages with its forces in the field. As this enabled the German military to act as a single co-ordinated entity, it gave them the basis for Blitzkrieg, the lightning war that overwhelmed Eastern Europe and then France. The code was thought to be unbreakable because it was transmitted and received via complex Enigma machines.

It immediately became a priority for the Allies to crack the code so they could monitor German troop movements, track their ships and submarines, and prepare for the Luftwaffe's strategy against the RAF and their bombing campaign on London.

The task of breaking Enigma was handled by teams at Bletchley Park. This is the story of how they exploited weaknesses in the machine itself and took advantage of errors made by the German operators. By cracking the code, the men and women at the park shortened the war by at least two years and saved five million lives.