Hitler's submarines, his "Grey Wolves," made their first appearance in 1939, and by 1942 had brought the Allies to the brink of defeat. Churchill would later admit, "The only thing that ever really frightened me during the War was the U-boat peril." At the most desperate hour, the capture of an Enigma machine and the invention of radar would prove instrumental in turning the battle, and therefore the war, around. This three-part series includes eyewitness accounts from former U-boat crew members and fascinating images of U-boats in action, along with dramatic reconstruction, to bring this crucial struggle for control of the "saltwater highways" into focus.