Spring 1943; the Allies are winning the war in the Atlantic thanks to Bletchley Park breaking the German submarine Enigma code. But suddenly, with three large Allied convoys sailing towards Britain, the Germans change the code. Lives and the outcome of the war hang in the balance. The brilliant cryptanalyst who broke the original code is brought in to do it again.
His ex-lover Claire disappears in highly suspect circumstances just as the security of the whole Enigma operation is at risk