March 1943, north of London: at Blechtley Park British cryptoanalists work on a top secret project. After getting hold of a German Enigma machine they have found a way to decode bits of the German military code. This allows for better protection of the convoys criss-crossing the Atlantic bringing in the desperately needed the supplies.
Just as a large allied convoy with ten thousand passengers and food supplies starts its way across the Ocean, the Germans change the code making it impossible for the British to foresee the routes of the German submarines.
Working at Blechtley is Tom Jericho, a gifted mathematician but emotionally fragile. He recently suffered a minor breakdown due to the pressure of his work but more so because of his unhappy relationship with Claire Romilly, a co-worker with whom he's fallen deeply in love. He returns from leave to help unravel the new code.
Unnoticed by his colleagues he also tries to solve a second problem: Claire has disappeared just as rumors spread that there is a spy at Blechtley Park and a file of German messages waiting to be decoded was found at her home.
Together with Claire`s room-mate Hester Jericho set out to find Claire and find out about the reason for her mysterious behaviour – under tight observation of secret service officer Wigram, assigned to ferret out the traitor, and another invisible enemy. Soon Tom and Hester discover a political conspiracy of international scale and personal revenge closely connected to Claire`s secret and the legendary Enigma.