In 1943, Lieutenant Commander MacClain (Randolph Scott) has just lost his ship and most of his crewmen due to enemy action. While accompanying a convoy, he was attacked by a U-boat with a distinctive large Iron Cross painted on the conning tower. The U-boat surfaced and machine-gunned many of the survivors. Offered duty ashore, MacClain is determined to avenge his men. He is allocated a new ship and while waiting for it to be built, befriends Joyce Cartwright (Ella Raines), whose brother Dick, an officer, was killed under his command.