The celebrated ‘Cambridge five’ have hitherto been believed to be the first long-term communist penetration agents in HM government, beginning with Donald Maclean in 1935. However, new research indicates that by 1919 another Cambridge man – like four of the ‘five’, a Trinity graduate – had already begun working for Moscow. This article is the first to examine how William Norman Ewer, known as ‘Trilby’ to his co-conspirators, organized networks in Great Britain and France to target the governments of those two powers. Under close Soviet supervision, Ewer's subordinates infiltrated half-a-dozen Whitehall departments, foremost among them Scotland Yard. Operating under the aegis of the home office, the Yard was a vital cog in the machinery of government set up to combat the ‘red menace’ in this country immediately after the First World War. By compromising the lead agency tasked with fighting them, the Bolsheviks thus created the requisite conditions for the metastasis in Great Britain of Soviet espionage in the 1920s.