In March 1943, cryptoanalysts at Bletchley Park, Britian's top code-breaking center, are facing their worst nightmare: Nazi U-boats have unexpectedly changed the code by which they communicate with each other and German High Command. An Allied merchant shipping convoy with 10,000 passengers and vital supplies is in danger of attack. The authorities turn for help to Tom Jericho, a brilliant young mathematician and code breaker, presently on sick leave. Unknown to colleagues, Jericho has another equally baffling enigma to unravel: Claire, the woman he loves, has disappeared just when the authorities suspect there may be a spy in Bletchley. To get to the bottom of both mysteries he enlists the help of Hester, Claire's best friend. Together they keep one step ahead of the secret services and investigate Claire's mysterious life, reaching a conclusion that uncovers international and personal betrayals.