Gordon Welchman was one of the original elite codebreakers crucial to the allies defeating the Nazis in World War II. He is the forgotten genius of Bletchley Park. Filmed extensively at Bletchley Park, the centre for codebreaking operations during World War II, this documentary features the abandoned buildings where thousands of people worked tirelessly to crack the codes. Post-war, Welchman moved to the US to be at the centre of the computer revolution. Recently released top secret documents reveal that the case of Gordon Welchman reached the desk of the British prime minister, Margaret Thatcher, and then led to questions being asked in the House of Commons after Welchman's untimely death. Welchman's legacy continues to this day. Welchman's pioneering work in the field of traffic analysis led directly to the modern secret surveillance state, and particularly the use of metadata - as revealed by whistleblower Edward Snowden.
1942, Nanjing (Nanking). Following a series of assassination attempts on officials of the Japanese-controlled puppet government, the Japanese spy chief gathers a group of suspects in a mansion house for questioning.