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. Their lives and the outcome of the war hang in the balance.
Tom Jericho, the brilliant cryptanalyst who broke the original code is brought from his sickbed to do it again. Meanwhile the security of the whole Enigma operation is at risk and Jericho's ex-lover Claire has disappeared in highly suspect circumstances.