Thousands of cattle killed mysteriously on the range, a series of strange accidents at the airbase at Twin Rivers, as Tom Mix seeks to solve these baffling mysteries, the man who was behind all of this - the Nazi saboteur known as the Iron Mask, prepares to strike one last powerful blow. Tonight at exactly 9 o'clock, he plans to kill ten thousand cattle, and at the same time bomb the hangar at the airbase, where Colonel Christie is carefully guarding a new secret fighter plane...