In March 1943 the code breakers at Bletchley Park, Britain’s top secret Station X, are facing their worst nightmare: Nazi U boats have unexpectedly changed the code by which they communicate with each other and German High Command. With an Allied merchant shipping convoy crossing the Atlantic with 10,000 passengers and vital supplies in danger of attack, the authorities turn to Tom Jericho, a brilliant young mathematician and code breaker, for help. But Jericho has another equally baffling enigma of his own to unravel, for Claire, the woman with whom he has fallen in love, has disappeared from Bletchley just when the authorities suspect there may be a spy at the Park. To get to the bottom of both mysteries Jericho enlists the help of Hester, Claire’s best friend. Together the pair must keep one step ahead of the secret services whilst investigating Claire’s mysterious life, reaching a shocking conclusion that uncovers both international and personal betrayals.