In 1963, Ennis Del Mar and Jack Twist are hired by Joe Aguirre to herd his sheep through the summer in Wyoming. After a night of heavy drinking, Jack makes a sexual pass at Ennis, who is initially apprehensive but eventually falls to Jack's advances. Though he informs Jack that it was a one-time incident, they develop a sexual and emotional relationship. Shortly after learning their summer together is being cut short, they briefly fight, and each is bloodied.
艾尼斯·戴尔·玛在怀俄明州与犹他州交界处的一个穷苦的小农庄里长大。他的父母死于车祸后他的哥哥和姐姐抚养他。父母为他们只留下了24美元现金和一个被抵押了两次的小庄园。艾尼斯没有读中学，他的少年是在劳苦和穷困中渡过的。他与艾尔玛·毕尔丝订婚，他不断地节省钱想以此买一点地。1963年，还不到20岁，他就在一个介绍农庄工作的公司里报到了。他来到了怀俄明的一个养羊的农场，在这里他结识了同岁的杰克·崔斯特。杰克已经在这个农场里工作了一个夏天了。这个英俊的、深色头发的、活泼爱笑的人特别热衷于当牛仔。两人接收了上千只母羊和羊羔，要将这些羊赶到断背山上没有树木的草原上去。

After Jack and Ennis part ways, Ennis marries his longtime fiancée Alma Beers and fathers two children. Jack returns the next summer, but Aguirre, who witnessed Jack and Ennis on the mountain, does not re-hire him. Jack meets, marries, and starts a family with rodeo rider Lureen Newsome. After four years, Jack visits Ennis. Upon meeting the two kiss passionately, which Alma accidentally witnesses. Jack broaches the subject of creating a life together on a small ranch, but Ennis, haunted by a childhood memory of the torture and murder of a man suspected of homosexual behavior, refuses. He is also unwilling to abandon his family. Ennis and Jack continue to meet for infrequent fishing trips.
艾尼斯留在宿营地，杰克每天晚上要与羊一起过夜，他必须每天好几个小时在营地和草原之间跋涉。两人意气相投，在篝火旁他们谈论马、驯牛和打架，很快就成了好朋友。两人都非常高兴在这里居然找到了这样的好朋友。艾尼斯甚至帮助杰克，与杰克轮流晚上去看羊。一晚他们酒喝多了，无法回到羊群所在地，艾尼斯就露宿在宿营地的篝火旁。半夜气温骤降，篝火也熄灭了，杰克就让艾尼斯和他在帐篷里挤一下。半梦半醒之间，他们擦出了火花，释放了一些积聚已久的感情。天亮后，两人都没有提起那夜的性爱，将它当作是一件离谱的事情。当然两人都不承认自己是同性恋。但很快他们就把放羊的事忘了，不但在夜里在帐篷里，就是在白天开满鲜花的草原上、傍晚在熊熊的篝火傍他们也互相爱着对方。八月，山顶飞雪，他们将羊群赶下山。断背山共同的时间结束了，艾尼斯非常悲伤，但他与杰克还是分手了。

The marriages of both men deteriorate. Alma and Ennis eventually divorce. Ennis sees his family regularly until Alma finally confronts him about her knowledge of the true nature of his relationship with Jack which incites a violent argument and causes Ennis to abandon his connections with Alma. Lureen abandons her rodeo days and becomes a businesswoman with her father and expects Jack to work in sales. Hearing about Ennis's divorce, Jack drives to Wyoming hoping they can live together, but Ennis refuses to move away from his children. Jack finds solace with male prostitutes in Mexico. Ennis meets and has a brief romantic relationship with a waitress, Cassie Cartwright. Jack and Lureen meet and befriend another couple, Randall and Lashawn Malone, and it is implied that Jack and Randall begin an affair behind their wives' backs.
艾尼斯回到艾尔玛身边，三个月后两人结婚了。他们移居到怀俄明州的瑞佛顿。四年后两人已经有了两个女儿，他早就不期望重见杰克了。就在这时他收到一张明信片，说杰克即将到瑞佛顿。艾尼斯急不可耐，穿上了他最好的衬衫去接他的好朋友。两人见面时的激情促使他们炙烈相吻，却没有想到艾尔玛在楼上的窗户边看到了这个景象，她感受到背叛和痛苦，难道彼此的夫妻感情也只是可笑的谎言？杰克在他们分别的第二年曾经回到打工的农场，希望再次遇到艾尼斯。后来他在德州成为牛仔，与一个富有工业家的女儿露琳结婚，并有一个八个月的儿子。杰克对他的儿子非常骄傲。虽然如此，他和艾尼斯还是喝醉了酒，在20分钟里就跑到了附近的一个汽车旅馆里去了。虽然两人炙爱，但他们都不想离弃妻子和儿女。尤其艾尼斯反对两人一起生活，他还记得他小时候看到一个同性恋的农工被人用一条汽车轮胎撬棍殴打至死，然后弃尸荒野。他们决定从此以后每年以一起钓鱼为由见两次面，共同度过一些秘密的幸福时光。艾尔玛知道艾尼斯的秘密，不愿再与艾尼斯一起生活。在艾尔玛容忍了六年之后，艾尔玛与艾尼斯离婚，与她打工的当地食品店的老板结婚。

At the end of a fishing trip, Ennis attempts to push back their next meeting. An argument erupts over Jack's frustration at him and Ennis meeting infrequently and Ennis blames Jack for being the cause of his own conflicted actions. Jack attempts to hold him and there is a brief struggle, but they end up locked in an embrace. A flashback of Ennis saying goodbye to Jack during their summer on Brokeback Mountain fades back to Jack watching Ennis drive away.
艾尼斯很快又开始做零工了，他在怀俄明的农庄间流荡。此后几年中他与杰克定期相遇，两人一起踏遍了附近的山峦河流，但却再也没有回到断背山。日月如梭，杰克越来越希望能和艾尼斯长相厮守，而艾尼斯则担心他们的结局而一再拒绝。杰克曾试图结交新朋友，也曾去过墨西哥嫖妓，但他却无法忘记艾尼斯。艾尼斯也遇到过喜欢他的姑娘，却总是无法爱上她。1983年，两人第一次相遇的二十年后，他们碰头时艾尼斯说下一次碰头要推迟，两人因此发生了争执，可还是友好地分别了。几个月后艾尼斯得到了杰克去世的消息。据说杰克在为轮胎打气时出了事故，车轮的辐条蹦出打在了他的脸上。艾尼斯想起轮胎撬棍的故事，他怀疑杰克的死不是一个事故。

Some time later, a postcard Ennis sends to Jack is returned stamped "Deceased". In a telephone conversation, Lureen tells Ennis that Jack died when a tire he was changing exploded; while listening, Ennis imagines Jack being bludgeoned to death by a gang. Jack's actual fate is left "deliberately ambiguous".[7] Lureen tells Ennis that Jack wanted to have his ashes scattered on Brokeback Mountain, but she does not know where it is. Ennis travels to meet with Jack's mother and father (Roberta Maxwell and Peter McRobbie), where he offers to take Jack's ashes – but the father refuses, preferring to have them interred in a family plot. In Jack's childhood bedroom, Ennis finds the bloodstained shirt he thought he lost on Brokeback Mountain, realizing that Jack kept it hanging with the bloodstained shirt Jack himself wore during that fight. Ennis holds them up to his face, breathing in their scent and silently weeping. Jack's mother allows him to keep the shirts.
杰克生前总是说要将自己的骨灰撒在断背山上，为此艾尼斯去杰克的父母家。在那里他听说杰克每年看他父母时都提到他要离婚，与艾尼斯一起搬到附近的一个小房子里，经营他父母的农庄。但他死前几个月他突然决定与他在德州的一个邻居一起来做这件事。因此艾尼斯猜到杰克的死可能的确不是事故。在杰克的房间里艾尼斯找到了两人第一次相遇时杰克所穿的牛仔衬衫，里面套著当时艾尼斯所穿的衬衣，象征著杰克深爱艾尼斯，深爱得像是杰克“包裹”著艾尼斯，二人紧紧的贴在一起。而衣衫的袖口还有他们分别时扭打的血迹，表示杰克从来没有忘记他俩在断背山美好的时光。当年艾尼斯以为这件衫已丢了，原来是被杰克拿走藏好了。杰克的父亲执意将杰克的骨灰葬在家族墓园里，艾尼斯不能圆杰克的心愿，但暗地里把两件衬衣带走。

Later, 19-year-old Alma Jr. arrives at her father's trailer with the news that she is engaged. She asks Ennis for his blessing and invites him to the wedding. Ennis asks her if her fiancé really loves her, and she answers "yes". After Alma's departure, Ennis goes to his closet. Hanging on a nail on the door are the shirts, with a postcard of Brokeback Mountain tacked above. Now Jack's shirt is tucked inside of Ennis's. Ennis fastens the top button of Jack's shirt, and with tears in his eyes mutters, "Jack, I swear..." while straightening the postcard, before closing the door and walking away.
最后，艾尼斯的大女儿来邀请他参加她的婚礼，两个相爱的年轻人可以走在一起。女儿走后，艾尼斯打开衣柜，柜门后钉著一张断背山的明信片，而两件老衬衣则挂在明信片下。不过，这次是艾尼斯的衬衣“包裹”著杰克的牛仔衬衫。艾尼斯轻声地说：“杰克，我发誓……”