During the height of World War Two, the whaler Daniel Webster sights a shell-damaged Danish schooner eerily adrift in the waters off Newfoundland. Unable to gain any response from the schooner's crew Pat Bannon (Dana Andrews) boards the helpless ship and finds its Captain (Claude Rains) unconscious. Deserted by his crew following their shelling by a German U-boat, Captain Skalder reluctantly agrees for Bannon to tow his vessel into port and make a claim for salvage. But as they head for Newfoundland, the Daniel Webster's radio is sabotaged. Suspicion falls on the whaler's sole passenger, the beautiful and mysterious Margaret McLean (Carla Balenda), and the rescued Danish captain; but as more accidents befall his ship, Bannon has to reluctantly accept that one of his crew is a Nazi secret agent.