Sheriff Tom Stocker sends for U.S. Marshal Johnny Mack to help him stop an outlaw reign of terror which is driving ranchers from Green Valley. En route, Johnny witnesses the murder of a messenger who is carrying secret information to the sheriff, and he and Stocker trail the killers to their hideout, where the sheriff is wounded in a gunfight and the killers get away. Stocker recognizes one of the men as Ed Rance, and Johnny watches him hoping to get a lead onto the identity of the gang leader. The news of a new secret rail route leads Johnny to check on who has being buying up land, and he learns that only stage-line owner Henry Faulkner and rancher Fred Kendal, father of the sheriff's sweetheart Margie, has recently purchased land. Faulker tells Johnny that Kendal has rounded up his entire herd, in order to sell it to purchase more land. Kendal is seriously wounded when the herd is stampeded by the outlaws, and Johnny gives out the news that Kendal is dead. Rance makes Margie an offer for the ranch on the behalf of a mysterious friend, but is taken captive by Johnny. The latter rides to town and asks Faulkner to come to the Kendal ranch to identify a possible outlaw suspect.