Katharine Hepburn and Spencer Tracy star as a husband and wife on America's frontier; farmers in a "sea of grass." The wife has an affair which produces a child, and she is ostracized from the community. Meanwhile, the farmers watch helplesslly as the sea of grass is exploited until it becomes a dustbowl.

St. Louis socialite Lutie Cameron is preparing for her wedding to cattle baron Colonel Jim Brewton, when Jim summons her to his vast ranch, Big Vega, near Salt Fork, New Mexico, claiming he is too busy to make the wedding. Despite cautious advice from her father, Lutie goes to Salt Fork and finds Jim and other cattle ranchers in the midst of a legal battle against a group of homesteaders, led by lawyer Brice Chamberlain. Jim, who owns nearly a million acres of prime land, some of it illegally, wants to maintain the territory for cattle grazing, insisting the land is too harsh for farming. Because of Jim's powerful influence, the ranchers win the court battle, but Brice vows that impending government regulations will soon force changes. Lutie and Jim have a quiet, simple wedding and Jim tries to impart his love of the land to her. Although Lutie quickly wins over the ranch hands and Jim's close friend, Doc Reid, she grows lonely for companionship and befriends Selina Hall and her homesteader husband Sam. Against Jim's advice, Lutie insists the Halls be given a small number of acres on Big Vega. Eventually Lutie gives birth to a daughter, Sarah Beth, but continues to feel lonely, as much of Jim's time is taken in managing the ranch. Lutie hosts several parties at Big Vega and grows friendly with Brice. One night during a harsh winter storm, Jim's men have difficulty driving the cattle back to the ranch and several stray into Sam's small homestead. Outraged at what he considers deliberate destruction, Sam defends his land and is beaten by the ranchers. Selina, several months pregnant, loses her baby when she tries to help her husband. When Lutie rides over later to offer assistance, the Halls reject her and soon after leave Salt Fork. Lutie protests the cruelty of the situation to Jim, who explains that the Halls' departure was inevitable. Miffed at his coldness, Lutie asks permission to go away for a time and Jim agrees. In Denver, despondent and troubled, Lutie runs into Brice, who declares his love for her and asks her to leave Jim. Carried away by Brice's attentions, Lutie spends the night with him, but the next day is filled with remorse, realizing she truly loves Jim. She returns to the ranch and promises Jim she will not interfere with decisions about the land again. Lutie tries to tell her husband about Brice, but only when she gives birth to a son, Brock, and while delirious mentions Brice does Jim learn the truth. Meanwhile, Brice is appointed judge of the Salt Fork territory and grants farming rights to homesteaders on Big Vega. Jim rounds up the ranchers and prepares for a range war, and when Lutie protests in favor of the homesteaders, Jim demands she leave Big Vega, without the children. Doc makes a futile attempt to bring Brice and Jim together to call off the range war, but Jim refuses, capitulating only when the U.S. Cavalry is sent to support the homesteaders. Lutie goes to St. Louis and keeps up with the children through letters from Doc. Two years later, she returns to Salt Fork to see her children and perhaps reconcile with Jim, but realizes her visit is a mistake and, despite Brice's pleas with her to stay, returns east. Meanwhile, the Salt Fork territory undergoes a severe drought, causing most of the homesteads to fail, as Jim had predicted. On his deathbed, Doc chastises Jim for his treatment of Lutie. Guilt ridden, Jim searches for Lutie, he but cannot find her. Over the years Lutie's children grow up, and Brice now writes to her about their upbringing. He describes how Jim dotes upon the insolent but charming Brock and how Sarah Beth's absence at school has only made Brock more wild. Concerned, Lutie starts back to Salt Fork. Meanwhile, Brock becomes involved in a gunfight with a gambler who taunts him about his parentage, and after Brice gets him out on bail, confides to Sarah Beth that he feels he must protect Jim from slander. When the gambler dies, Brock, realizing a public trial would humiliate Jim, jumps bail and flees. Sarah Beth tells Jim the reason for Brock's actions and Jim goes after him, finding him fatally wounded in a cabin surrounded by deputies. Brock dies in Jim's arms just as Lutie arrives in town. Sarah Beth confronts her mother about the past, but realizes Lutie genuinely loves Jim and takes her home to the ranch. There Lutie is at last reconciled with Jim.