Summer 1943. A mysterious alien offers Hitler a way to defeat the Allies...for a price. The crew of American submarine USS Barb, on patrol in the English Channel, engages in a running battle with a German U-boat. US Navy codebreaker Barbara Moore, beautiful and courageous, intercepts communications between the aliens and the Nazi High Command. Together with the crew of USS Barb, Moore must find a way to stop the alien invasion of the British Isles...