At 8 p.m. on 5 September 1945 Igor Gouzenko, principal cipher clerk to Colonel Nikolai Zabotin, the military attaché at the Soviet embassy in Ottawa, Canada, defected. Over the previous few weeks he had removed from the embassy a number of documents marked ‘Top secret. Burn after reading.’ In total there were more than a hundred carefully selected secret documents detailing a vast Soviet network operating in the United States and Canada, among them a member of the British high commissioner's staff, Kathleen Wilsher, codenamed ‘Ellie’. The British high commissioner, Malcolm MacDonald, was informed of the case and it was decided that Ellie should be placed under observation. MacDonald then agreed to serve on a small informal committee made up of Canadian intelligence officers, the Canadian Prime Minister, Mackenzie King, and the Canadian Permanent Secretary for External Affairs, Norman Robertson, to investigate the extent of Soviet espionage in Canada. MacDonald's membership of this committee was regarded as essential, since Mackenzie King ‘wished to share all the secrets with his colleagues in London’.

As more and more of the Gouzenko material was translated it became clear that the reach of the Soviet spy networks in the US and Canada was bigger than had first been anticipated and included the Canadian National Research Council. Within the first few days of Gouzenko's defection four agents working for the council were exposed as spies. These were Durnford Smith, E. W. Mazerall, Dr Raymond Boyer and a fourth agent identified only by his codename ‘Alek’, known to be closely associated with the manufacture of the atomic bomb. There was also a fifth man, Israel Halperin, whose level of involvement remained hidden. The identity of Alek, however, was discovered after a telegram from Nikolai Zabotin to Moscow Centre was intercepted, revealing that Alek would soon be taking up an appointment as a lecturer at the University of London: