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It is March 1943 and inside Britain's codebreaking centre, Bletchley Park, the cryptanalysts are facing their worst nightmare: Nazi Germany's U-boats have unexpectedly changed their Enigma cipher and the Battle of the Atlantic suddenly hangs in the balance. 

In a desperate attempt to save the situation, the authorities turn for help to Tom Jericho, a brilliant young mathematician and codebreaker, presently on sick-leave in Cambridge. What follows is a frantic race to crack the code of the U-boat. But unknown to his colleagues, Jericho also has an equally baffling enigma of his own to unravel: the woman he loves has disappeared and he suspects there may be a spy in Bletchley…

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