In Wyoming in 1963, cowboys Ennis Del Mar and Jack Twist are hired by foreman Joe Aguirre to herd his sheep through the summer on grazing pastures on Brokeback Mountain. After a night of heavy drinking, Jack makes a pass at Ennis. While initially reluctant, Ennis becomes receptive, and he and Jack have sex in their tent. Despite Ennis telling Jack that it was a one-time incident, they develop a sexual and emotional relationship. Before parting ways, Ennis and Jack have a brawl that leaves both of them bloodied. Ennis later marries his longtime fiancée Alma Beers and has two daughters with her: Alma Jr. and Jenny. Jack returns the next summer seeking work, but Aguirre, who had observed Jack and Ennis engaging in homosexual activity on the mountain, refuses to hire him again. Jack moves to Texas, where he meets wealthy rodeo rider Lureen Newsome; they marry and have a son.
After four years apart, Jack visits Ennis. Upon meeting, the two passionately kiss, which a stunned Alma inadvertently witnesses. In the privacy of a motel room, Jack broaches the subject of creating a life with Ennis on a ranch, but Ennis refuses, as he is unwilling to abandon his family and is haunted by a childhood memory of his father showing him the body of a man who was tortured and killed for suspected homosexuality. Ennis and Jack continue to infrequently meet for private fishing trips while their respective marriages deteriorate. Lureen abandons the rodeo and goes into business with her father, which causes Jack to work in sales. Alma and Ennis divorce in 1975. Upon hearing about the divorce, Jack drives to Wyoming and tells Ennis that they should live together, but Ennis refuses to move away from his children. Upset, Jack finds solace with male prostitutes in Mexico.
Alma takes custody of Alma Jr. and Jenny, and marries Monroe, manager of the grocery store where she works as a clerk. Ennis visits them during a Thanksgiving dinner. In the kitchen, when Alma and Ennis are alone, she confronts him about the true nature of his relationship with Jack. The two spar, causing Ennis to storm out and cease contact with Alma. Ennis has a short-lived romantic relationship with a waitress named Cassie. Jack and Lureen befriend another couple, Randall and Lashawn Malone, and it is implied that Jack and Randall have a brief affair. At the end of a fishing trip, Ennis tells Jack that he cannot see him again before November due to work demands. The pair argue, blaming each other for their not being together. Ennis begins to cry, and Jack embraces him.
Sometime later, Ennis receives a returned postcard that he had sent to Jack, stamped with "Deceased". Ennis calls Jack's phone number, and Lureen picks up the call. She tells Ennis that Jack died in an accident from drowning in his own blood after a car tire exploded in his face. While she is describing what happened, Ennis envisions a group of men brutally beating Jack to death with a tire iron. Lureen tells Ennis that Jack wanted to have his ashes scattered on Brokeback Mountain, and Ennis visits Jack's parents in hopes of carrying out that wish. Jack's father refuses and contends that Jack's ashes will be interred in a family plot. Jack's mother tells Ennis that he is welcome to go to Jack's bedroom upstairs, if he wants. Ennis goes there alone, and in the closet, he finds the shirts which the two cowboys wore during their brawl long back, with Ennis's shirt inside Jack's. Ennis holds the shirts to his face, inhales deeply, and silently weeps. When Ennis returns downstairs, Jack's mother sees him holding the two shirts, and allows him to keep them.
Later, a 19-year-old Alma Jr. arrives at Ennis's trailer to tell him that she is engaged to Kurt, a man who works in the oil fields. She asks for his blessing and invites him to the wedding. Ennis briefly hesitates due to his work commitments, but then he agrees to attend the wedding. Once Alma Jr. leaves, Ennis goes to the closet where the two shirts hang together, though now Jack's shirt is inside Ennis's. Next to them, tacked to the closet door, is a postcard of Brokeback Mountain. With tears in his eyes, he stares at the mementos, and says, "Jack, I swear..."
