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 a train called The Prairie Queen.
Soon, Sheriff Quin Milam is hot in pursuit of Mari, but not for the reason one might think. It turns out that the dead poker player has left a four-year-old orphan behind -- and the sheriff has decided Mari should take responsibility for her.
But circumstances on the Prairie Queen are heading towards catastrophe. The train is halted and the passengers are taken hostage by the Cherokees, in an event that becomes an incident of major social and political consequence.