Under the command of British Major John Smith, an international commando unit dressed in Nazi uniforms parachutes into the Bavarian Alps with the objective of rescuing a World War II Allied officer from a German castle-fortress known as the Castle of the Eagle. After one of the commandos is found with his neck broken, Smith first rendezvouses with agent Mary Ellison and then with another agent, Heidi, who, by posing as a local barmaid, succeeds in getting Mary into the fortress by introducing her as a cousin. Following the murder of a second commando and the capture of three other men, Smith and his second-in-command, American Ranger Morris Schaffer, are forced to surrender. But they escape and gain entry into the fortress by crouching on the roof of a cable car. Once there, Smith reveals to Schaffer that the Allied officer is really an actor, and that the real purpose of the mission is to discover the identity of German spies in England. After the actor-imposter has been rescued, Smith tricks the three "captured" commandos into exposing themselves as German agents. Now that they have the vital information they were sent for, Smith, Schaffer, Mary, and the imposter must face the problem of escaping. Having wired the fortress as well as certain sites in the village to explode at intervals, they make their way to a cable car. Once aboard, they outwit the Germans, reach the village, leap off the cable car, and, with the enemy still in hot pursuit, make their way through a canal to a garage. There they commandeer a bus and drive to an airfield where a plane is arriving to take them back to England. Once they are in flight, Major Smith forces one of the mission's organizers, Colonel Turner, into confessing that he is a German agent. To prevent humiliation to Turner and his family, as well as a national scandal, Smith permits Turner to meet a more honorable death by allowing him to leap from the plane.