In 1940, almost a year after the outbreak of the Second World War, Allied radio operators at an interception station in South London began picking up messages in a strange new code. Traffic in this new encryption increased dramatically and Bletchley Park codebreakers worked furiously to decipher the code that held the key to the secrets of Nazi high command. The codebreakers used science, maths, innovation and improvisation to invent an entirely new machine: Colossus. Colossus was Instrumental in several extraordinary breakthroughs: It revealed that Hitler had no intention 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 couldn't deploy tanks in time); It monitored the locations of Nazi military troops in northwestern and southern Europe; It revealed the degree to which Germans has succeeded in breaking Allied codes What these codebreakers didn't realize was that they had fashioned the world's first true computer. When the war ended, this incredible invention was dismantled and hidden away for almost fifty years. With access to previously classified files, Paul Gannon has pieced together the tremendous story of what is now recognized as the greatest secret of Bletchley Park.

Paul Gannon is a writer on all aspects of information and communications technology. He is the author of Trojan Horses & National Champions: A History of the European Computing and Telecommunications Industry (IT Book of the Year, 1997) and Colossus (Atlantic, 2006).