The action of the film starts with Colonel Dodge arriving on the first train and subsequently opening the new railroad line that links Dodge City with the rest of the world. A few years later, Dodge City has turned into the "longhorn cattle center of the world and wide-open babylon of the American frontier, packed with settlers, thieves and gunmen -- the town that knew no ethics but cash and killing". In particular, it is Jeff Surrett and his gang who kill, steal, cheat and, generally, control life in Dodge City without ever being brought to justice. As Surrett has installed one of his puppets as sheriff, the other citizens' hands are tied when it comes to arresting any of the evildoers.
Wade Hatton, a lone cowboy who was instrumental in bringing the railroad to Dodge City, is now on his way to the town leading a trek of settlers from the East coast. At Hatton's side is his old companion Rusty, who is prepared to stay with him through thick and thin. Among the settlers are beautiful Abbie Irving and her irresponsible brother, who, drunk, causes a stampede and is shot by Hatton in self-defense. When the group arrive in Dodge City, Hatton is confronted with the full extent of the anarchy which is dictating everyday life there. Asked by anxious citizens -- Abbie's uncle among them -- to be the new sheriff, Hatton politely declines, saying he is not cut out for this kind of job.
Hatton changes his mind when, during a school outing, a young boy is inadvertently killed by Surrett and his men. The new sheriff and his deputy -- Rusty of course -- have a hard time not just fighting the criminals but also convincing all the farmers who have been wronged by Surrett that mob rule ("Come on, boys, let's take 'em out to the plaza") is out of the question: When Yancy, one of Surrett's thugs, is in jail, Hatton has to protect him against the furious men outside who, not caring for Yancy's right to a fair trial, want to take the law into their own hands and lynch him right then and there.
In the end Hatton succeeds in both overwhelming and catching the baddies and winning Abbie's heart. Everything has been prepared for a quiet family life in newly civilized Dodge City when news reaches Hatton of another town farther West which also desperately needs cleaning up. Obedient and so much in love, Abbie agrees to her husband's change of plan and is willing to join him and Rusty on their new mission.