The Portland Spy Ring was one of the most notorious spy cases from the Cold War. It seized international attention and revealed the shadowy world of deep cover KGB spies operating under false identities ('illegals'). The CIA's revelation to MI5 that a KGB agent was stealing crucial secrets from the sensitive submarine research base at Portland in Dorset looked initially like a dangerous but contained lapse of security by a British man and his mistress. But the unsuspecting couple passed the secrets to a Canadian businessman, Gordon Lonsdale. Lonsdale in turn led MI5's spycatchers to an innocent-looking couple in suburban Ruislip called the Krogers, who were transmitting the vital information to Moscow. A sudden defection forced the arrest of the spy ring. The Krogers were discovered to be two of the most important Russian 'illegals' ever. The Americans had been searching for them for years.

In a previous undercover life they had been a conduit to the KGB atomic spies at Los Alamos. And Lonsdale was no Canadian, but a senior KGB controller called Konon Molody - who years later turned out to have been running other key Soviet agents in the UK. This astonishing but true story of MI5's spyhunt is straight from the world of John le Carré. It criss-crosses the world between the USSR, USA, Canada, New Zealand, Europe and the UK, and ends with dramatic spy swaps familiar to viewers of Steven Spielberg's Bridge of Spies . This extraordinary story is a fascinating vignette of the Cold War stand-off between the USSR and the West, and a case that fully justified the West's paranoia about infiltration and treachery.