In the Montana territory, in 1871, sheepherders invade land occupied by cattle ranchers.  Morgan Lane, a sheepman whose father was driven off his land by cattlemen, returns to Montana, determined to graze sheep.  After one of his herders is killed, Morgan accompanies Poppa Schultz, a traveling salesman, into town, and he presents himself as Poppa's partner.  The opposition to the sheepmen is led by cattle ranchers Maria Singleton, whose father was killed in a sheep and cattle war, and her fiancé, Rod Ackroyd.

Still pretending to be a salesman, Morgan asks the ranchers about their hostility to sheep.  His questions raise the suspicions of cattle rancher Slim Reeves, who challenges him to a gunfight.  Later, Tecumseh Burke, an old cowhand, privately reveals to Morgan that he knows his real identity.

Maria invites Morgan to dinner with Rod and George Forsythe, who owns the land adjoining Maria's ranch.  During the evening, Morgan bets Rod a portion of his land that he can ride any horse on the property.  Rod selects a horse that is reputed to be a killer and, to make sure that Morgan loses, cuts the stirrup.  Even so, Morgan almost wins his bet.

Afterward, Maria offers to lease some land to Morgan, whom she still believes is interested in running cattle, and makes it clear how strongly she is opposed to sheep.  After Morgan's intention to bring sheep onto the land is revealed, however, Reeves severely beats and tries to kill Morgan.  Tecumseh saves Morgan and later explains that Reeves's fury stems from the fact that the small cattlemen are being squeezed out of business by the large ranchers.  Morgan meets with them and suggests that they run their cattle along with sheep.

Later, he tries to bring Forsythe in on their side by pointing out that the price of beef is declining while the price of wool is high.  Forsythe is interested, but does not want to go against his neighbors, so he invites Rod and Maria to meet with Morgan and the other ranchers to work out a solution.  Rod declines, however, partly because he plans to take advantage of Maria's absence to meet another woman, who is also the sheriff's girl friend.  Maria and Forsythe argue about the pros and cons of allowing sheep on the land, and most of the ranchers support Maria.  When Forsythe announces that he will allow sheep on his ranch, Reeves kills him.  In retaliation, Morgan kills Reeves and a range war starts.

The sheriff, who witnessed Rod's rendezvous with his girl friend, unofficially warns Morgan that Rod and Maria are planning to stampede their cattle and drive the sheep away.  After a shootout, the sheriff, Morgan and his associates stop the stampede, but Rod is trampled to death.  Maria then takes a stand against the sheep and wounds Morgan while trying to stop them from entering the town.  She instantly regrets it, however, and as she and Morgan embrace, concedes that sheep and cattle ranchers can live together.