The study of native `Marxisms' in Britain throws light on what many historians have referred to as `the enemy within'. In this book, David Burke looks at the activities of the Russian political emigre community in Britain, and in particular the role of one Russian-Jewish political family: the Rothsteins. Theodore Rothstein and his son Andrew, along with his sister-in-law Zelda Kahan and her husband, W. P. Coates, together played an important part in the formative years of the Communist Party of Great Britain and were closely monitored by the British secret service. This led to claims that British communism was effectively a Russian creation with Theodore Rothstein acting as the eminence grise; the hidden hand of Moscow controlling the British left-wing movement. In 1920 Theodore Rothstein's activities on the left of the British labour movement assisted the formation of a Communist party in Britain affiliated to the Comintern. Theodore was, soon after, effectively debarred from Britain following a visit to Russia, at which point his clandestine political activities passed to his son, Andrew. This book encompasses two periods. The first looks at the contribution of Theodore Rothstein to British Marxism and the response of the British intelligence services, Special Branch and MI5, to what they regarded as a serious threat to British security. The second part probes Andrew Rothstein's subsequent career, and considers four main events: the formation of the Anglo-Russian Committee in 1924, the Zinoviev Letter, the General Strike of 1926 and the ARCOS Raid of 1927, and concluding with Andrew Rothstein joining his father in Moscow in 1930. With access to recently released documents from MI5, this book sheds new light on the activities of British Marxists against the backdrop of the early twentieth century and brings to life the story of a remarkable family.