or years, Britain had no idea that people working at the very heart of its intelligence system were secretly serving the Soviet Union. From the halls of Cambridge to the inner circles of MI6, the spy network known as the Cambridge Five quietly passed thousands of classified documents to Moscow, exposed agents, sabotaged counterintelligence operations, and triggered one of the biggest espionage scandals in British history.
How did they infiltrate the system? And why did no one uncover them for decades?