This somewhat off-the-beaten-path-formula for a Saturday matinée B-western has Johnny Mack Brown being hired by stageline owner Chet Norman to stop a series of stagecoach holdups that always take place when the driver, Pete, sees a mystery rider and hears the weird notes of a silver whistle. Sheriff Dave Holland, in love with Chet's niece, Beth Fairchild, and jealous of Johnny, gives grudging help. Town banker Roger Claine, is the mastermind behind the gang led by the Mystery Rider and Slade, and is having an affair with waitress Cora, 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.