This is the story of Robert Hanssen - a former FBI agent who systematically dismantled the United States’ network of undercover Soviet agents, from the inside out. His espionage was described by the Department of Justice as "possibly the worst intelligence disaster in U.S. history”. Hanssen spied for Soviet and Russian intelligence services against the United States from 1979 to 2001 and his actions pitted CIA against FBI and triggered a mole hunt that brought the American intelligence services to their knees.