The Cold War had been over for two decades when, in 2010, ten Russian agents were arrested in the United States after a decade-long FBI operation. Among the spies were three couples who had lived as Americans for years, and one agent who had lived undercover for so long that he could barely even speak Russian. They had hidden their true identities from their children, neighbours and even their partners. The Illegals captures the untold history of Russia's deep cover spy programme, following its evolution from the talented "great Illegals" of the 1920s and '30s up to the 21st century, when agents maintained their fake identities and loyalties even after the fall of the Soviet Union. Through hundreds of interviews and access to never-before-seen archives, Shaun Walker exposes the colourful history of the KGB's most secretive espionage project.