On their way to Brimstone, Arizona in 1879, boxer Jeffrey Burton, known as "The Kanarsy Kid," and his trainer, Francis X. "Eightball" Harrigan, are robbed by a masked bandit who, among other things, takes Jeff's pocket watch, which holds a picture of his late mother. When the bandit, "Trigger" Bill, inspects the watch, he recognizes the picture as that of his long-abandoned wife and realizes that Jeff must be his son. Though he doesn't want Jeff to know their relationship, Bill decides to look out for his son in the lawless town of Brimstone. Jeff doesn't respond well to Bill's attempts at protection, and Bill, in turn, finds Jeff's antagonism irritating. When Jeff falls in love with Loretta Douglas, the daughter of Judge Jackson Douglas, however, Bill decides to help him by having Judge Douglas give Jeff $3,000 for his college education, saying that it is an inheritance from his deceased father. Bill confesses the truth to the judge, but makes him promise never to tell Jeff or anyone else in town their real relationship. On the day that Jeff leaves, Bill meets him and Eightball on the road and says goodbye, but only allows his friend, Portuguese Ben, to know how much he really cares for his son, and tells the town's toughest outlaw, Chester "Blackjack" McCreedy, that he has killed Jeff and Eightball for causing the death of one of Blackjack's men. That same day, miners come to town revealing that a large silver vein and been discovered, and Brimstone soon becomes a boom town. Three years later, a mature Jeff returns to town as a lawyer and the new United States Marshal. He soon discovers that during his absence Brimstone has become larger and more lawless, and that Bill and Blackjack run the town. Judge Jackson, who has become weakened with drink and age, has become Blackjack's pawn and bows to his wishes whenever any of Blackjack's cohorts are brought to trial. When Blackjack ruthlessly kills "Eightball" after learning that Bill lied about killing him and Jeff, the judge only sentences Blackjack to $500 or one year in jail after he is convicted of murder, insensing the town's honest citizens. Jeff then takes action to rid Brimstone of Blackjack and his crowd and warns Bill that he can no longer operate as he has in the past. In an ensuing gunbattle, Portuguese Ben is killed, as is the judge, who makes one last attempt at honesty by refusing the cooperate with Blackjack and handing over to Jeff a book that documents all of Blackjack's crimes. When Bill comes to Jeff's aid against Blackjack, Jeff offers him amnesty, but Bill cannot accept because he knows that he must kill Blackjack to protect himself and Jeff. Soon Bill begins his pursuit of Blackjack and Jeff is forced to post Bill as a wanted criminal. After some time, Bill finally tracks down and kills Blackjack. He then returns to Brimstone to see Jeff one last time and discovers that Jeff and Loretta are being married. He is captured while watching the ceremony from outside the church, but convices his captor to allow him to stay just until the end of the ceremony.
