Keeley Hawes narrates the story of a secret Second World War mission in which a 24-year-old maths student and a GPO engineer tried to crack Adolf Hitler's personal code machine - reputed to be even more secure than the Nazis' renowned Enigma code. The duo's project led to one of the major breakthroughs of the war and played an important role in ending the conflict in Europe, but because it remained a secret long after the fighting had ended, the two men who devised the plan were never officially recognized.