During and after the Second World War, a group of elite, highly educated British gentlemen passed thousands of classified documents to the Soviet Union, so many that the KGB suspected it had to be a trap. This episode constructs a collective biography of the Cambridge Five and how they reshaped the Cold War from the inside out.

We trace them from their privileged academic origins through their staggering infiltration of the British establishment to their dramatic unmasking and the decades of institutional cover-up that followed.

• The ideological incubator of 1930s Cambridge, the Great Depression, rising fascism, and Anthony Blunt’s role as a talent spotter for Soviet intelligence.
• How Blunt, Guy Burgess, Donald Maclean, Kim Philby, and John Cairncross ascended into MI6, MI5, the Foreign Office, the Treasury, and Bletchley Park.
• The industrial-scale theft of more than 16,000 documents and how affinity bias blinded investigators to spies within their own social class.
• The Venona Project decryptions, the mole codenamed Homer, and the 1951 defection of Burgess and Maclean to Moscow.
• Philby’s escape to Russia, the secret confessions and immunity deals for Blunt and Cairncross, and the government’s long campaign to avoid public embarrassment.