An Arizona bandit gang led by Jim Rivers repeatedly robs the federal mail, but when sheriff Big Bill Collins follows Lucky Webster from the scene of the robbery to his ranch, Lucky is provided an alibi by Tom Allen and his sidekick, Lopez Mendoza. Although Tom and Lopez are strangers to Lucky, he hires them to be his new ranch hands, and Lucky's sister Julia takes Tom into her confidence. The next day, Tom wanders onto a trail marked "keep off," and ends up being chased by two bandits. Tom rides right into the clutches of Rivers, but when Lucky says he knows Tom, Rivers agrees to cut Tom in on his next deal. Julia, who is concerned about her brother, later beseeches Tom to discover the reason for Lucky's recent erratic behavior. Unknown to Julia, Tom and Lopez are undercover marshals working in collaboration with the sheriff to rid the county of the bandits. Tom soon surmises that bank president Jeff Gordon is the mastermind behind the robberies, because Gordon would profit twice: when the bank's money is stolen by his gang from the mail service, he pockets the money as well as the postal service's insurance rebate. With the help of Lopez's miniature monkey, "The General," Tom installs a secret listening device in Gordon's office, and the sheriff listens in on his conversations. Bill and Tom overhear Gordon plotting his next robbery, in which he plans to send $15,000 through the mail and have Rivers dynamite the train and steal the money. Tom and Bill arrange for a posse to meet the bandits at the only site possible for such a robbery, but Julia overhears their plans and warns her brother. Tom and Lopez are ambushed when they return to the hideout and are tied up. Rivers plans to dynamite the cabin, killing Tom and Lopez and destroying all the evidence, but The General takes the dynamite outside, where it explodes safely, and then releases his human companions. Tom, having overheard Rivers' plans to outwit Gordon and steal the money from the post office, intercepts the gang, and he and the sheriff's posse chase them to Lucky's ranch. Lucky is killed in the ensuing gun battle, and Tom is forced to shoot Rivers in self-defense. The bandits' reign is ended as Bill arrests Gordon. In order to ease her conscience, Tom tells Julia that Lucky died while helping him, then he rides off with Lopez.