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When a gunman, on the run from the law, comes upon a dying U.S. marshal, Matt Brennan, he takes his badge and adopts his identity. In the nearby town of Banock, Dr. Steve Hale fears that he is facing an outbreak of an epidemic when an Indian's son dies under mysterious circumstances. The town derives most of its meat and dairy products from the herds owned by cattle baron Col. Doniphan and his son Shad, and Steve suspects that their cattle are possible carriers of a disease. With Sheriff Carl Smith's help, Steve takes a blood sample from one of Doniphan's calves and his lab work confirms that the animal is infected with black spot fever. Steve asks Carl if he can arrange for him to examine the entire herd, but that exceeds Carl's authority and requires an order from a federal marshal. Meanwhile, Doniphan shows Shad several dead cattle and, fearing that his entire herd will be ordered destroyed, tells him to begin a roundup to remove all the cattle from the ranch. The colonel also orders Shad to help Chicamon, the Indian who blames Steve for the death of his child and intends to kill him. Shad organizes a lynching and is about to hang Steve when "Brennan," who is riding by, stops the hanging at gunpoint, arrests Shad and delivers him to the jail in Banock. As Doniphan puts up bail for Shad, several of his gunmen rough up Steve, who is rescued by Carl and Brennan. After more townspeople fall ill, Carl, Steve and Murdock, the female owner of a small café, plead with Brennan to stay and declare an official emergency, but Brennan does not want to get involved, saying that he intends to move on. When Carl receives a "wanted" poster for a Jeff Harlan, whom he recognizes as Brennan, he says nothing. Eventually, Brennan decides to stay and agrees to help Steve to take more blood from the sick cattle, which have not yet been rounded up, in an attempt to create an antidote. Meanwhile, Doniphan uses his considerable political influence to summon the governor to his ranch to prevent the slaughter of his herds. As more people fall ill and die, Murdock converts her café into a hospital. Although she is falling in love with Brennan, he is unwilling to make any commitment. During Doniphan's roundup, Felipé, one of his ranchhands, collapses with the fever and Shad shoots him. Steve, Brennan and Carl find Felipé and, before he dies, he tells them that the governor is arriving to see Doniphan, who still intends to drive the diseased cattle elsewhere. Brennan wants no further involvement in the town's problems and again attempts to leave, but is persuaded by Carl to stay a little longer to help him intercept the governor and plead for his help. While they wait for the governor, Carl shows Brennan the "wanted for murder" poster and Brennan tells him that the murder was a fair fight, but he never was given a chance to prove it. After Carl stops the governor and explains the situation, the governor authorizes him to halt the cattle drive. When Carl returns to Banock, he discovers that Steve's antidote is working, prompting many of the grateful townspeople to volunteer to help Carl stop the drive. Brennan has not returned with Carl, so Murdock tracks him down and, after telling him that Carl has told her of his past, convinces him that they can start a new life together. The next morning, Brennan joins Carl, Steve and the townsmen in a gunfight as the Doniphans attempt to drive the herd across a river. After Brennan shoots the colonel and beats up Shad, the ranchhands turn back with the cattle. Brennan then decides to settle down with Murdock in Banock.