Lady gambler Mari Marshay decides to give it all up after a gunfight at her poker table kills a man. Leaving a note for the sheriff to locate the man's widow and return the deed to his ranch which was among her winnings, she boards the train called The Prairie Queen. Sheriff Milam has an urgent reason to catch up with Mari: the man has left a four-year-old orphan. But circumstances on that train are heading towards catastrophe. Eventually the train is halted and the passengers are taken hostage by the Cherokees in what would become an incident of major social and political consequence.