U.S. Marshal Len Merrick and his deputies Billy Shear and Lou Gray stop the lynching of rustler Pop Keith, whom rancher Ed Roden believes is the killer of his son Ed, Jr. Despite the vehement protests of Roden and his younger son Dan, Merrick insists on bringing Keith to trial in the distant town of Santa Loma. Professing his innocence, Keith begs Merrick to let him escape. As it is almost dark, Keith then persuades Merrick and his deputies to spend the night at his nearby farm. When Merrick spots Roden removing his son's body from the scene of the murder, he sends the others on to the Keith farm and joins Roden, who reiterates his belief that Keith is the killer. After Roden leaves the area, Merrick finds a watch, which he takes with him. Near the Keith farm, he hears gunshots, and approaches cautiously. He discovers that Keith's daughter Ann is attempting to free her father and disarms her. Later, she makes a second unsuccessful try to help Keith escape. Outside the house, Merrick encounters some cattlemen, who warn the marshal that Roden will kill all of them. Merrick, his deputies, Keith and Ann then leave immediately for Santa Loma along the shorter desert route. When Keith sings the song, "Down in the Valley," Merrick angrily orders him to stop, but after Billy reveals that Merrick's father used to sing the song, Keith realizes he has found a weak spot and continues singing. Later, during a rest stop, Keith taunts Merrick, asking if he reminds the marshal of his own father, which causes Merrick to attack the older man. Soon Merrick spots Roden and his men, and the group takes shelter behind some rocks. During the ensuing gunfight, their main source of water is destroyed, and Billy is wounded. Merrick then captures Dan and takes him along as a safeguard against future attacks. Even though Billy is badly wounded, and they have only the water in their canteens, they continue across the desert. When Billy dies, Ann denounces Merrick for his unyielding devotion to the law. To justify himself, Merrick divulges the one time he forgot the law: Once, acting as his father's deputy, he left his father alone to deliver two prisoners for trial because he thought they deserved to be lynched. Unable to stop a lynching party, his father was hanged along with the prisoners. On the arduous journey, the group endures lack of water and a sandstorm. Finally they reach a water hole, only to discover that the water has gone bad. A heat-crazed Dan insists that his father poisoned it in order to kill him. Keith points out that a river lies only a half-day's ride to the south, but aware that the river marks the Mexican border, Merrick refuses to turn in that direction. The others rebel, and as insurance, Merrick forces Ann to share his horse. Finally, exhausted, Merrick's horse stumbles, and Keith grabs his gun. Dan encourages Keith to kill the marshal, but instead, when Gray grabs a rifle, intending to take control and head for water, Keith shoots and kills the deputy. The remaining members of the group continue on foot to Santa Loma, where Merrick turns Keith and Dan over to the sheriff. Keith's trial is held that night. Merrick, who has begun to doubt the fairness of the law, speaks in Keith's favor, but despite his efforts, Keith is found guilty and sentenced to hang. The next morning, a distraught Ann accuses Merrick of failing to make sure he had arrested the right man. Disgusted, Merrick pulls off his badge, and the watch falls from his pocket. Seeing the inscription "To Dan" inside, he realizes that Dan killed his brother and confronts the Rodens. After accusing Roden of giving Ed everything while giving him nothing, Dan shoots at his father and runs off. Merrick then kills Dan, and a grateful Ann embraces him, causing Keith to express his dismay at the prospect of a lawman in the family.