Rooster Cogburn, an aging, one-eyed U.S. Marshal, has his badge ripped away by Judge Parker in Fort Smith, Arkansas, after years marked by heavy drinking and a history of volatile firearm use. A daring theft sets the story in motion when a shipment of high-explosive nitroglycerin is stolen from a US Army transport by the ruthless bank robber Hawk and his nine-outlaw gang. Cogburn is offered a chance at redemption: pursue the criminals, recover the nitro, and prove his worth.
His pursuit passes into the Indian Territory, where he traces the gang to a desolate settlement that once was a town called Fort Ruby and has since become a Christian mission. He arrives to find the village laid waste by the raid, the elderly Reverend Rev. Goodnight killed, and many locals slain or displaced. The only survivors are the Reverend’s spinster daughter, Eula Goodnight, and a Native student named Wolf, a teenage boy whose family died in the attack. Cogburn would rather leave them at a nearby trading post, but they insist on joining him in a bid to capture the gang.
With the nitro needing careful, slow transport to prevent detonation, Hawk pushes ahead with Breed and an injured ally to prep for the bank raid. Cogburn, along with Eula and Wolf, closes in on the rest of the gang and blocks their path through a narrow gully by stacking logs as a barricade. When the outlaws pause, Cogburn declares himself the head of a posse and issues an ultimatum: let him arrest them, or risk the nitro exploding. An outlaw attempts a surprise shot from behind, but Eula Goodnight proves a superb marksman, picking off the attacker from across the debris-strewn gully. Cogburn then fires on another member, while Eula and Wolf create noise by firing into the air, causing the outlaws to retreat and abandon the wagon carrying the nitro—and the Gatling gun that sits aboard it. The trio reunites with Hawk, who is brought back to the ambush site, where Breed reveals that Cogburn’s boast of leading a large posse may have been a bluff.
The gang resumes the chase, catching up to Cogburn’s party under the cover of night. They kidnap Wolf, who had dozed on his post, and demand Wolf in exchange for the explosives. Wolf shoots the kidnapper holding him and makes it back to Cogburn and Eula, enabling a daring escape with the nitro-laden wagon after Wolf scatters the outlaws’ horses to slow pursuit.
A river crossing forces Cogburn to improvise a raft from an old ferryboat operated by Shanghai McCoy. The nitro and Gatling gun are stashed aboard as the trio travels downstream, intent on placing safety between themselves and Hawk’s gang while they seek a safer landing point where the rest of the pursuit can be wound down. Breed and Luke ride the heels of the raft, waiting for a moment to exploit Hawk’s plan. Luke suggests a double-cross, but Breed remains loyal to Cogburn, even at the cost of his own life, killing Luke when the betrayal is suspected and warning Cogburn about Hawk’s intent. Breed then rejoins the gang, though Hawk remains unconvinced by Breed’s claims about Luke’s fate, and he hurls Breed off a cliff to his death.
Cogburn uses the Gatling gun to hold the gang at bay, but the river narrows into treacherous rapids. The group rides through the churning waters, losing the Gatling gun in the process. Ahead, the gang’s horses can be heard, so they drop several boxes filled with nitro overboard to propel a makeshift charge toward the pursuers. Eula and Wolf delay Hawk by feigning a surrender and convincing him that Cogburn is injured, buying time for Cogburn to strike. He targets the floating nitro boxes and detonates them, eliminating the remaining outlaws in a final, explosive act.
Back in Fort Smith, Eula negotiates with Judge Parker to reinstate Cogburn despite the earlier condition to bring Hawks back alive. With the danger behind them, Eula and Wolf set about finding families to resettle Fort Ruby and, in a quiet, hopeful moment, part ways with Cogburn, leaving open the possibility that their paths might cross again someday.
