Jim Flagg is the marshal in the town of Progress. He hears arch-rival Big John McKay is headed toward town so he warns Mayor Wilker and others in Progress about rumor of an impending train robbery. Wilker doesn't appreciate Flagg causing a panic and relieves him of his job and badge by retiring him.
Flagg sets out on his own and discovers McKay has joined up with a band of youthful outlaws. After being taken prisoner, Flagg escapes death thanks to McKay's intervention after the youthful Waco takes over the gang, but the two old enemies end up in a bloody fistfight.
Taken back to town, Flagg puts McKay in a boarding house run by Mary, a widow. The mayor and town folks don't take the threat seriously. When the outlaws arrive, intent on robbing a train, McKay sides with Flagg in defeating their plans.
The outlaws intend to rob the train before it arrives at the town bank, when it stops at the station. Flagg and McKay board the train before it reaches town. Although they are initially detained by on-board security inside a privy, they are able to break free. They climb into the engine room and take the workers hostage. The train does not stop at the station and passes into town.
Mayor Wilker and a band of townspeople chase after the train. The outlaws, upon realising the train has not stopped, also chase after it. McKay detaches the front compartments of the train from the rear passenger carriages, overtaking the outlaws. The train approaches a section of broken track over a cliff and is too fast to stop in time. Flagg, McKay and the workers bail out before it plunges off the cliff and explodes.
The outlaws catch up and rob the burning train compartments. Flagg and McKay ambush them and kill most of them in the ensuing gunfight. McKay meets Waco as he is about to escape. Waco manages to wound McKay, who then shoots him dead. McKay remarks to Flagg that he "thought he could beat him [Waco]" to which Flagg replies "You did beat him."
Mayor Wilker arrives and expresses his gratitude to the two for saving the town and hence his reputation as a mayor. When interviewed by a press journalist, Wilker is asked whether he would consider running for state governor. He is delighted to consider the prospect. Flagg thinks Wilker really could "become president one day."
The town marshal asks Flagg to take back his badge for his heroic deed, but he turns down the opportunity. He gives one piece of advice to the marshal, that, in order to be successful, "You have to tell the good guys from the bad guys."
The film ends on a humorous note as Flagg arrests McKay and handcuffs him, despite McKay's protests. Referring to an earlier incident in the film, Flagg jokes that he will always keep his word, as he once promised to land McKay in jail.
