Using recently declassified information, this is the greatest untold story of Bletchley Park: a gripping account of the invention of the world's first true computer, Colossus, and the crucial part it played in winning the war.

Colossus was instrumental in several crucial breakthroughs:

• It revealed that Hitler had no intentions of abandoning Italy to the Allies
• It revealed how much the Nazis had been duped by the D-Day deception (they believed the Allies would invade at Calais and were tactically unprepared for invasion at Normandy, so were not able to deploy tanks in time)
• It monitored the locations of Nazi military troops in north-western and southern Europe
• It revealed the degree to which Germans had succeeded in breaking Allied codes.