The Maclean brothers, Norman and Paul, grow up in Missoula, Montana, with their mother, Clara, and their father, Rev. John Maclean, a Presbyterian minister, from whom they learn a love of fly fishing for trout in the Blackfoot River. Norman and Paul are home-schooled under the strict moral and academic code of their father. Norman leaves to attend college at Dartmouth. When he returns six years later during the Prohibition era and the Jazz Age, he finds that Paul has become a highly skilled fisherman and a hard-drinking, fearless investigative journalist working for a newspaper in Helena.
Norman attends a Fourth of July dance and meets Jessie Burns, a flapper whose father runs the general store in Wolf Creek. Immediately smitten, Norman calls her the next morning and sets up a double date. Norman and Jessie go on their first date at the Hot Springs speakeasy. Paul arrives with his date, a similarly hard-drinking Cheyenne woman named Mabel, who is treated as an inferior by the local white crowd.
Soon after, Norman is called to bail Paul out of jail after Paul is arrested for hitting a man who insulted Mabel. The desk sergeant tells Norman that Paul has angered local criminals by falling behind in his debts from a big poker game at the Lolo speakeasy. Norman offers to give Paul money, but Paul brushes him off.
After Norman and Jessie go on several dates, she asks him to help her alcoholic brother Neal, who is visiting from Southern California. Norman and Paul dislike Neal, but at Jessie's insistence they invite him to go fly fishing. Neal shows up drunk with Rawhide, a prostitute whom he met the night before. Norman and Paul get separated from Neal but fish anyway and return to their car hours later to find that Neal and Rawhide have drunk all the beer, had sex, and passed out naked.
Norman drives an intoxicated Neal home, where Jessie is enraged that the brothers left Neal alone with the beer instead of fishing with him. Norman tells her that he is falling in love with her. Jessie drives away angry but a week later asks Norman to come to the train station to see Neal off. After the train departs, Jessie laments her failure to save Neal from his alcoholism and asks why the people who need help the most will not accept it. Norman shows Jessie a letter from the University of Chicago offering him a faculty position in the Department of English Literature. He tells Jessie that he does not wish to leave Montana and when it becomes clear that it is because of her, she embraces him.
That night, a drunken Norman meets up with Paul and announces his love for Jessie. Paul says they should celebrate but instead takes Norman to the Lolo speakeasy. Paul tries to get in on the poker game in the backroom, but the dealer will not let him play because he already owes so much. Paul tells Norman that he isn't leaving since he is feeling lucky and that he will convince the others to let him play. Norman reluctantly drives off after Paul asks him to go fishing the next day.
Blackfoot RiverNorman is relieved when Paul arrives the following morning, as he feared for his brother's life. Norman tells his family that he is going to accept the job in Chicago. Norman, Paul, and their father go fly fishing one last time. Norman urges his brother to come with him and Jessie to Chicago, but Paul says he will never leave Montana. He hooks a huge rainbow trout that drags him down the river rapids before he lands it. Their father tells Paul that he has become a wonderful fisherman and an artist in the craft, much to Paul's delight.
Just before Norman is to leave for Chicago, police inform him that Paul was beaten to death. Norman breaks the news to his parents. Years later, Mrs. Maclean, Norman, Jessie, and their two children listen to a sermon given by Rev. Maclean soon before his own death. Rev. Maclean preaches about being unable to help loved ones who are destroying themselves and will not accept help. All that those who truly care for such a self-destructive person can do, Rev. Maclean concludes, is to give unconditional love, even without understanding why.
The closing scene shows an elderly Norman Maclean fishing on the same river as director Robert Redford narrates the final lines from Maclean's original novella.
