Soviet Colonel Oleg Penkovsky was one of the highest-ranking Soviet officials ever to spy for the West, and he risked his life providing an unparalleled amount of information to MI6 and the CIA. At the height of the Cuban Missile Crisis, Kennedy could turn to technical evidence unknown to the rest of the world provided by "Agent Hero" – Penkovsky's codename. One of the most effective spies in MI6's history, Penkovsky soon realised the KGB were on his tail.

With unprecedented access to KGB archives, the film shows the surveillance footage taken by the KGB as they trailed Penkovsky across Moscow in meetings with his British handlers – Janet Chisholm, a British diplomat's wife, and Greville Wynne, a British businessman. Declassified CIA transcripts reveal that as America was being targeted, so was Penkovsky. His dramatic story and tragic end is highly revealing of KGB operations at the height of the Cold War.