Under Ten Flags is an action-filled war drama based on a true story. Original footage and actual war ships lend realism to the film. Captain Bernhard Rogge (Van Heflin) is in command of the German Naval vessel Atlantis, a Nazi Raider, whose mission is to destroy as many Allied Naval ships as possible without being detected. After several successful attacks, British Admiral Russell (Charles Laughton) is tasked with investigating who is responsible and coming up with a plan to stop the Nazi Raiders who are sinking British ships. Aided by Knoche (Alex Nicol), an American spy who is able to steal intel from the German ship, Russell works to stop the Atlantis. Romance, rescued children, and clever disguises complicate matters for both the Nazis and the Brits. The ships' captains and crew must show courage, strength, and cunning to complete their missions, but only one side can emerge the victor.