 This somewhat off-the-beaten-path-formula for a Saturday matinee B-western has Johnny Mack Brown (Johnny Mack Brown) being hired by stageline owner Chet Norman (I. Stanford Jolley)to stop a series of stagecoach holdups that always take place when the driver, Pete (Bud Osborne), sees a mystery rider and hears the weird notes of a silver whistle. Sheriff Dave Holland (James Ellison as Jimmy Ellison), in love with Chet s niece, Beth Fairchild (Noel Neill), and jealous of Johnny, gives grudging help. Town banker Roger Claine (Marshall Reed), is the mastermind behind the gang led by the Mystery Rider and Slade (Lee Roberts), and is having an affair with waitress Cora (Pamela Duncan), and has promised her he will kill Slade when they have enough money. Beth is shot by Dave in an aborted holdup that reveals her to be the Mystery Rider, working with Claine in a spirit of revenge, thinking that her uncle had stolen the stageline from her father.