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. An Allied merchant shipping convoy crossing the Atlantic with 10,000 passengers and vital supplies is in danger of attack. The authorities turn for help to Tom Jericho (Dougray Scott), a brilliant young mathematician and code breaker.