Colossus, the world's first electronic computer, was built during World War II, but kept secret for more than 30 years. Professor Brian Randell tells the story about how he stumbled across a reference to its existence and eventually led to the UK government lifting the veil of secrecy surrounding this pioneering computer in 1975. Prof Brian Randell's presentation was given in the new Colossus Gallery in The National Museum of Computing on 7 February 2013.