A prim schoolteacher turns outlaw queen when the railroad steals her land.

Catherine Ballou, an aspiring schoolteacher, is traveling by train to Wolf City, Wyoming, to visit her rancher father, Frankie Ballou. En route she unwittingly helps accused cattle rustler Clay Boone elude his captor, the sheriff, when Boone's Uncle Jed, a drunkard disguised as a preacher, distracts the lawman. She reaches the ranch to find that the Wolf City Developing Company is trying to take away the ranch from her father, whose only defender is an educated Indian, Jackson Two-Bears. Clay and Jed appear and reluctantly offer to help Catherine. She also wires legendary gunfighter Kid Shelleen to come and help protect her father from fast-drawing Tim Strawn, alias Silvernose, the hired killer who is threatening Frankie. Shelleen arrives, a drunken stumblebum who is literally unable to hit the side of a barn when he shoots and whose pants fall down when he draws his gun. Strawn kills Frankie, but the townspeople refuse to bring him to justice, and Catherine becomes a revenge-seeking outlaw known as Cat Ballou. She and her four associates rob a train carrying the Wolf City payroll, and Shelleen, inspired by his love for Cat (unrequited because she loves Clay), shapes up and kills Strawn. Later he casually admits that Strawn was his brother. Cat poses as a prostitute and confronts town boss Sir Harry Percival, owner of the Wolf City Developing Company. A struggle ensues; Harry is killed; and Cat is sentenced to be hanged. Just as the noose is being placed around her neck, however, her gang arrives and stages a daring rescue.