England 1943. Much of the infamous Nazi Enigma code has been cracked. But Shark, the impenetrable operational cipher used by Nazi U-boats, has masked the Germans' movements, allowing them to destroy a record number of Allied vessels. Feeling that the lives of Allied sailors are in their hands, 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 have somehow learned that Shark has been cracked. And they've changed the code. . . .

As an Allied convoy crosses the U-boat infested North Atlantic . . . as Jericho's ex-lover Claire 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. . . .