The action takes place in Montana in the 1880s among the tributary streams that eventually flow together into the Missouri River, an area known as the Missouri Breaks. Tom Logan, (Jack Nicholson) is the likable leader of an amiable gang of horse thieves. They make their living stealing horses or rustling cattle from ranchers, laughing all the way, spending their money on whiskey and whores.Logan and his gang are particularly upset by the hanging of a friend by rancher David Braxton (John McLiam), an egomaniacal gay local land baron who took the law into his own hands. Braxton has horses, and a daughter, worth stealing.Logan's men pull off a daring train robbery, only to lose much of the money. The gang retaliates against Braxton by hanging his foreman Pete Marker (Richard Bradford) on the same noose.Posing as an aspiring cattle rancher, Logan buys an abandoned ranch next to Braxton's to serve as a relay station for moving stolen horses across the plains.While his men ride off across the Missouri River and north of the border to steal Canadian Mounted Police horses, Logan plants crops at their new farm and enters into a relationship with Braxton's virginal daughter, Jane (Kathleen Lloyd). He knows how to court a woman by brewing Chinese tea in the Wild West. Jane is on bad terms with her father and has taken a shine to Logan in any event, seems to be fascinated by the bad guys and demands sex, saying: "Do you want to have sexual intercourse?" while flirting with Logan. Braxton, obsessed with his rustling problem, his daughter, and avenging his friend's murder, sends for Robert E. Lee Clayton (Marlon Brando), a notorious regulator from Medicine Hat, Wyoming, who, for a price, will take care of rustlers personally, one at a time. Regulators were mercenary frontier agents famous for their ruthlessness and their ability to kill without warning from long distances with their trademark Creekmore rifles. Clayton arrives with a fancy wardrobe, a perfumed scent, an Irish brogue and a rifle with which he is deadly accurate from afar. Clayton bursts in on the funeral of the murdered foreman dressed like a western dandy in fringed leather coat and scarf, bellowing and yanking the corpse up out of the open casket to borrow a few of the ice cubes used to keep the body from decomposing as a compress for a tooth-ache.Clayton is portrayed comically, but he is a sadistic, perverse, and psychotic killer who comes across as much more lawless than the criminals he's hunting.Quickly suspicious of Logan, who doesn't strike him as much of a farmer, Clayton dons a variety of disguises to pick off Logan's gang, one by one.Impersonating a preacher "Jim Ferguson," Clayton tries to cajole Little Todd into confessing that he is part of the gang. He kills Todd by dragging him with a rope through the raging Missouri, seemingly aware of the fact that Todd cannot swim.Clayton spies on Logan with binoculars and taunts Braxton about his daughter's affair with a horse thief. Braxton attempts to discharge him but Clayton is determined to finish what he starts.He amuses himself by shooting two more of Logan's partners from a distance and then by wearing a granny dress while brutally killing Logan's closest friend, Cal (Harry Dean Stanton) with a handmade weapon.Logan knows it's kill or be killed by Clayton. He also wants vengeance against Braxton for having hired the regulator in the first place, despite his feelings for Jane. Now it's up to Logan to hunt down Clayton. One night after a campfire goes dark with Clayton serenading his horse, Logan gets his revenge catching up with him while he's asleep and then slitting his throat: "You know what woke you up? You just had your throat cut!" Logan then comes after Braxton. There's a shootout where Braxton, who everyone now believes is infirm with a stroke, pulls out a gun while he's sitting in a wheelchair and shoots the unawares Logan. Logan then shoots back killing Braxton.Logan abandons his farm and packs up to leave the area and Jane, although he acknowledges that he and Jane might renew their relationship another time, another place.