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. Using science, maths, innovation and improvisation Bletchley Park codebreakers worked furiously to invent a machine to decipher what turned out to be the secrets of Nazi high command. It was called Colossus. 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 50 years. Paul Gannon has pieced together the tremendous story of what is now recognized as the greatest secret of Bletchley Park.