Ennis del Mar and Jack Twist are two poor boys born on small ranches in Wyoming around the time of the Second World War. Ennis’ parents died when he was young, so his older brother and sister raised him. Both boys dropped of out of school in order to work, and meet for the first time in the summer of 1963 as ranch hands on Brokeback Mountain, before either of them is 20.
Their boss is Joe Aguirre, a cold man who doesn’t think much of the two boys when he meets them. Ennis is assigned the job of tender, and Jack is given the role of herder, for which Joe commands him to sleep far out in the wilderness to watch the sheep at night.
Jack has worked at the mountain before, and he gives Ennis advice about working there. The two boys get to work, occasionally noticing each other across the natural beauty of the mountain.
As the summer wears on, they get to know each other well, exchanging stories about their families and backgrounds. Jack complains that he hates being so far away from the main camp, and Ennis offers to switch roles with him.
One night, after Ennis and Jack have been up late drinking and talking, Jack declares that it’s too late for Ennis to go back out to the sheep, and that Ennis should stay with him. Soon they begin to have sex, an encounter that neither of them speak about but implicitly know will repeat itself through the rest of the summer.
In the wilderness and solitude of the mountain, Ennis and Jack feel free and alone in their intimacy. However, Joe Aguirre watches them have sex through binoculars, and treats Jack coldly when he delivers the news that his uncle is dying. There is a hailstorm, and the sheep from the Brokeback herd get mixed up with another. Ennis is unable to accurately pick them apart.
The summer finishes, and Jack and Ennis go their separate ways: Ennis to marry Alma Beers, and Jack back home to Lightning Flat. When they say goodbye, Ennis feels sick to his stomach.
Ennis marries Alma, and she quickly becomes pregnant, giving birth to a daughter they name Alma Jr. Ennis works a series of low-paying ranch jobs. He enjoys their itinerant lifestyle, but Alma hates it. Four years after the summer of 1963, Ennis receives a postcard from Jack, saying he’ll be in town. When the men reunite, they kiss passionately, which Alma sees. They spend a few days together in a motel room, where they admit how much they had missed each other, and resolve to see each other more often. Jack wants them to get a ranch together, but Ennis thinks it would be too conspicuous. He tells Jack about Earl and Rich, two ranchers who lived together near Ennis when he was young. Earl was beaten to death with a tire iron and mutilated. Ennis’ father made sure Ennis saw the corpse, and Ennis surmised that his father had something to do with the death.
As the years wear on, Jack and Ennis continue to steal time to see one another under the guise of “fishing trips.” Alma and Ennis have another child that they name Francine, but their marriage eventually falls apart and Alma gets remarried to the local grocer. Ennis attends Thanksgiving at her new home with the grocer, where Alma accuses him of having a relationship with Jack. Ennis responds violently and doesn’t see his children for many years after that.
Ennis continues to work low-paying, itinerant ranch jobs. Jack marries Lureen, a wealthy woman whose father runs a farm-machinery business. They continue to see each other over the years, and their desire does not wane, even as their bodies age. In 1983, they spend several days together at a lake. Jack expresses his frustration that they don’t see each other more often. Ennis replies that he has work responsibilities, and needs to pay child support. He is also still scared about their physical safety when they are together. They leave things unresolved.
Several months later, Ennis sends a postcard to Jack about arranging their next meeting. It is returned with the notice DECEASED. Jack calls Lureen, who tells Ennis that Jack died when a tire exploded while he was changing it. Ennis assumes that there is more to the story than this, and that Jack likely died in a manner similar to Earl.
Ennis visits Jack’s family, and asks to scatter Jack’s ashes on Brokeback Mountain, like Lureen mentioned he had wanted. Jack’s father is cruel to him, and refuses, insinuating that he knows Jack and Ennis were lovers. In Jack’s room, Ennis finds a shirt that Jack wore on the Mountain, and nested inside it, one of his own shirts that had gone missing. He takes these home with him and hangs them on the wall of his trailer alongside a postcard of Brokeback Mountain. He resigns himself to dreaming about what could have been between the two of them, and continues to live his life on the fringes of society.