This is the incredible tale of Operation Fifth Column, a Second World War MI5 operation so secret that its existence was revealed by the National Archives for the first time only in 2014.

Throughout the war and even for a couple of years afterwards, 'Agent Jack' - in reality, a bank clerk named Eric Roberts - acted as a Gestapo agent to whom hundreds of British-based Nazi sympathisers and informers passed their secrets, thinking that he was sending them back to Germany. Many were put on a salary by what they thought was the Third Reich, and some were even 'awarded' Iron Crosses for their services to the Fatherland; they never found out the truth.

Among the secrets they tried to pass were: a tip-off about Bletchley Park; details of the deadly Mosquito bomber; and complete plans of a highly effective antiradar technology code named WINDOW. The larger-than-life characters who populate the book include Roberts himself, the deceptively ordinary-seeming bank clerk; Maxwell Knight, who recruited Roberts; Victor Third Baron Rothschild, Roberts' spymaster, who did a sideline in bomb disposal using his Cartier screwdrivers; Theresa Clay, the distinguished biologist who co-ran the operation with Rothschild, but because she was a woman was only ever classified as an 'assistant'; and Marita Perigoe, possibly the most dangerous of the fascists, who despite having her suspicions about Roberts, continued to recruit spies for him and pass him secrets to the end of the war.