For almost half a century, Bertram Mills Circus was a household name throughout Britain among both children and adults and it's Director, Cyril Bertram Mills, was one of the best-known and most influential names in the country's entertainment business.

But for forty years, Cyril Mills had also enjoyed a top-secret and wide-ranging career in British intelligence: obtaining the best aerial intelligence on Nazi rearmament for MI6 before the Second World War; becoming the first case officer to monitor the best double agent (Garbo) of the war after joining MI5; and working part-time during the Cold War 'for MI5 or 6 or both without being paid a penny'.

Remarkably, no word of Mills's secret career appeared in public until he was over eighty. Nobody suspected that the glamorous world of pre-war circus entertainment had been an extraordinarily fitting rehearsal for the lethal arena of deception and surveillance. In this remarkable true story, Christopher Andrew, best-selling official biographer of MI5, brings to life one of the most surprising and fascinating tales of espionage ever told.