After the Second World War, many of the men and women who had worked at Bletchley Park moved on to GCHQ: the British government's new facility established to fight the KGB. The Spies of Winter explores the early years of GCHQ as it navigated its way through a tumultuous era—from the defection of the Cambridge Five and the treachery of atomic scientist Klaus Fuchs to the collapse of the British Empire and the emergence of the US as a superpower. This is the story of the codebreakers themselves, and how they expanded the horizons of cryptography, helping to defend the nation and maintain the fragile peace in a world now under the shadow of nuclear holocaust.