In 2010, two decades after the Cold War, ten Russian spies were arrested in the US following a ten-year FBI operation. Among them were three couples who had lived as Americans for years, and one agent who had nearly forgotten Russian. They had hidden their true identities from their children, neighbours and even their partners.

Moscow expert Shaun Walker captures the untold history of Russia's deep cover spy programme, from the 'great illegals' of the 1920s and 1930s to the twenty-first century, when agents maintained their fake identities and loyalties after the fall of the Soviet Union. Through hundreds of interviews and access to never-before-seen archives, The Illegals brings to light the KGB's most secretive and surprising espionage project ever.