Atlantic Convoy

During World War II, near a U.S. Marine flying patrol station in the waters off the coast of Iceland, a series of torpedo attacks on Allied supply ships leads Captain Jim Morgan to suspect that information about the location of the ships is being leaked to enemy submarines. Gregory, one of the Marine pilots, immediately suspects German-born weatherman Carl Hansen, but Morgan does not believe that Hansen is responsible for the attacks. When another freighter, the Mary Heather , is torpedoed, Morgan sends Hansen with Gregory to search for survivors. Among those rescued from the torpedoed ship are nurse Lida Adams, five English refugee children and Sandy Brown, a young German agent posing as a British cabin boy. When all the survivors are safely loaded onto the amphibious plane, a piece of a raft hits the plane, sending Gregory flying across the cabin and knocking him unconscious. Though not trained as a pilot, Hansen manages to fly the plane back to the base with ease, a feat that astonishes Morgan and leads him to suspect that Hansen is an impostor. Meanwhile, Sandy, the actual source of the leaks, secretly visits Otto, a fellow Nazi spy who operates a phony fishing company in the area. Hansen later comes to suspect Sandy when the boy inadvertently blurts out a German phrase while watching one of the rescued British children die. Hansen, who has fallen in love with Lida, tries to persuade her that Sandy is the spy, but she now suspects Hansen. When Morgan's investigation of Hansen turns up the fact that he deserted the Marines just before a court-martial hearing, and that he was accused of delivering top secret information to the Germans, Morgan goes to the weatherman's house to arrest him. Hansen escapes through an open window, however, and a manhunt ensues. While taking refuge aboard Otto's fishing boat, Hansen soon confirms his suspicions about Sandy, and becomes his captive. Hansen recognizes Otto from his past association with the Nazis, when the German tricked him into betraying his country. Otto and Sandy coerce Hansen into rigging their fishing boat so that it can be operated by remote control to ram and explode an Allied ammunition ship. Although Hansen outsmarts his captors by secretly alerting Allied ship inspectors that Otto's fishing boat is carrying live ammunition, he is still on board when Morgan orders the aerial bombardment of the boat. Otto and Hansen escape by getting into a bruising fistfight that propels them into the water moments before the fishing boat is bombed. Otto is eventually captured, and Lida, now realizing that Hansen is innocent, rushes into his arms.