England 1943, much of the infamous Nazi Enigma code has been cracked. But Shark, the impenetrable cipher used by Nazi U-boats, has masked the Germans' movements, allowing them to destroy Allied vessels. Now a top-secret team of British cryptographers works feverishly around the clock to break Shark. And when brilliant mathematician Tom Jericho succeeds, it is the stuff of legend.

Until the unthinkable happens: the Germans change the code. As an Allied convoy crosses the U-boat-infested North Atlantic....as Jericho's ex-lover disappears amid accusations that she is a Nazi collaborator.......as Jericho strains his last resources to break Shark again, he cannot escape the ultimate truth: There is a traitor among them.