In the blistering heat of the Bitter Water Valley, Jim Deming serves as the sheriff of Calor, earning the nickname "Duty" for his unyielding adherence to the law. His uncompromising nature has already fractured his family, leading his wife to leave him after he imprisoned their son based on questionable evidence. Despite losing his deputy and running the office alone, Deming immediately locks away a sick hobo found stealing a ride on a passing train. Shortly afterward, news reaches the town that a cattleman named Mitchell has been murdered and a cowboy named Red Cowan has fled into the treacherous lava beds.
Driven by his rigid sense of obligation, Deming rides out alone into the waterless waste without blankets or adequate preparation. The desert sun and the early stages of a contagious illness soon take their toll, leaving the sheriff delirious and collapsing near a deserted ranch. There, he is miraculously found by Red Cowan, who spends days nursing the feverish lawman with remarkable care and compassion. Completely unaware of his host's identity or the true purpose of his manhunt, Cowan tends to the sick sheriff in the isolated adobe house.
As Deming’s mind clears enough to recognize his surroundings, the obsession with his office returns to dominate his thoughts. Mistaking a calf or hallucinating the threat, and terrified that his quarry might escape, the weakened sheriff arms himself and shoots at the man who saved his life. The bullet strikes an animal rather than the cowboy, but the exertion pushes Deming's compromised heart past its limit just as his former deputy, Joe Mills, arrives with a search party.
Before passing away, Deming points out the man he believes is the murderer, only to learn a stunning truth from Mills. The actual killer was another cowboy named Delong, who has since died after a separate accident and confessed to the murder as well as planting the evidence that framed the sheriff's son. Furthermore, the illness that brought Deming down was not smallpox, but common measles contracted from the hobo he so strictly arrested. As the men prepare to transport the fallen lawman back to town, the narrative reflects on the tragic irony of a life consumed by an unyielding devotion to duty.
