1881 Arizona is a hard place to live, and a place where the Law seems to have no foothold.
From the East comes the Ellis family: recently widowed Ada and her daughters, Caroline and Adeline, as well as her granddaughter Damaris. Inheriting a ranch and a newspaper, they dig in to make a life in the desert.
Just outside of Dragoon, Jack Hartman is beaten and left for dead by Big Don Norris and his men. Found by Marshal Ben Shaw, he recovers, to become the sheriff of Sundown City. Jack means to bring Law to the town, while Adeline brings the newspaper back to life. U.S. Marshal, Ben Shaw, mourning the loss of his wife and child back in Texas, is brought face to face with Caroline Stewart, the girl he loved before the War. Trying to leave the past of an unfaithful husband behind her, Caroline is confronted by feelings for a man she long thought dead.
Meanwhile, Adeline, determined to be dependent on no man and with a desire for a career, makes sure the sheriff doesn't have matrimony on his mind.
This is a time when guns and might make right and Big Don Norris and his men have claimed Sundown City for their own. With the railroad running through it, he has in mind an empire that stretches from Tucson to Benson to Tombstone. And he knows just the man to run it; himself.
The past will have to be forgotten as the lawmen, townspeople and ladies of business, unite to bring law and order to the Southwest and they have Until Sundown.