NOTE: This spoiler was submitted by Blake.
14-year-old Mattie Ross (Hailee Steinfeld) arrives at Fort Smith, Arkansas to have the body of her murdered father sent back to the family farm. Tom Chaney (Josh Brolin), a former farmhand to her father, got drunk one night and shot her father dead in a confrontation before fleeing on horseback to Indian Territory with two stolen California gold pieces in his possession.
Mattie makes inquiry around town about finding a U.S. Marshal willing to go after Chaney and finally settles upon Reubeun “Rooster” Cogburn (Jeff Bridges) after hearing of his reputation for being a ruthless lawman. Attending a trial in which Cogburn testifies and it is made apparent that he has something of a quick trigger when it comes to dealing out justice, Mattie attempts to hire him to hunt down Chaney but Cogburn shows little interest in doing business with her.
Upon returning to the boarding house where she has been forced to sleep for the duration of her stay in Fort Smith, Mattie is greeted by a cocky Texas Ranger by the name of La Boeuf (Matt Damon), who has been on the hunt for Chaney for several months and seeks to bring him back to Waco, Texas to be hanged for shooting a senator. After some verbal sparring, La Boeuf departs when Mattie refuses to help him, as she intends to have Chaney hanged in Arkansas.
Mattie finds Cogburn sleeping in the back of a Chinese grocery and finally convinces him to agree to a fee of $50 up front for his help in tracking down Chaney. But, the next morning, Mattie finds that Cogburn and La Boeuf have struck a deal and left without her. Refusing to give up, Mattie crosses a river on horseback and convinces Cogburn to let her accompany them after threatening to have him brought up on charges of fraud for stealing her money, much to the chagrin of La Boeuf.
Later on, Cogburn and La Boeuf get in an argument that ends with the two of them going their separate ways, leaving Mattie to ride with Cogburn. The two of them happen upon an isolated shack where two outlaws are hiding out and, after interrogating them both about the whereabouts of Chaney and his gang, is forced to shoot them both when one stabs the other for giving up information.
Finding a hiding spot nearby, Cogburn and Mattie wait for “Lucky” Ned Pepper (Barry Pepper), Chaney, and their outlaw gang to arrive. Instead, La Boeuf shows up and searches the shack just as Pepper and three other outlaws arrive on horseback. When one of the gang members lassos La Boeuf and starts dragging him around, Cogburn fires on the gang and kills three of them, with Pepper escaping on his horse. La Boeuf has been shot in the shoulder and his tongue has almost been bitten off but he, Cogburn, and Mattie spend the night in the cabin.
Finding bottles of whiskey in the shack, Cogburn proceeds to get drunk as the three of them continue on their trek to find Pepper, Chaney, and the gang. Eventually, the trail runs cold and Cogburn drunkenly admits that he is giving up, prompting La Boeuf to depart on his horse. Mattie begs La Boeuf not to leave her but, while he admits that she has earned his respect, he too agrees that the trail has been lost.
The next morning, Mattie goes to fetch some water from a nearby stream but is shocked to find Chaney tending to three horses in the same stream. He attempts to approach her but she shoots him in the arm using her father’s gun, wounding him. Now visibly angered, Chaney grabs her as her gun misfires and drags her away, just as Cogburn appears and opens fire on him. Pepper and his gang fire back at Cogburn and demand that he leave the area if he wants Mattie to live.
After watching Cogburn ride off, Pepper and three other outlaws leave on horseback, promising to return soon for Chaney and Mattie. Once they depart, Chaney admits to Mattie that he has essentially been left for dead and intends to kill her. Putting a knife to her throat, Chaney is dealt a blow to the head by La Boeuf, who heard the gunshots down at the stream and made a plan with Cogburn to rescue Mattie.
Down in the valley below, Pepper and his three cohorts find themselves face to face with Cogburn, who warns Pepper that he has come to take him dead or alive. Pepper and his gang charge on Cogburn and he is able to gun down three of them and wound Pepper before his horse shot and pins him to the ground. Just as Pepper is about to shoot Cogburn, La Boeuf shoots him from atop his perch and kills Pepper.
Mattie is ecstatic until Chaney regains consciousness and smashes La Boeuf over the head with a rock. Grabbing La Boeuf’s rifle, Mattie shoots Chaney in the chest and kills him, only to fall back into a cavern filled with snakes. Cogburn is able to come to her rescue but not before she is bitten by one of the snakes, forcing him to cut open the bite and suck out of the venom. With only one horse and Mattie in need of medical attention, Cogburn and Mattie are forced to leave La Boeuf behind. Eventually, their horse collapses under the strain of their long return journey and Cogburn is forced to shoot it, proceeding to carry Mattie until they manage to find a doctor.
Thirty years later, Mattie, who had part of her arm amputated after the snake bite, attempts to reunite with Cogburn at a traveling circus where he has become a performer. Much to her dismay, Mattie is informed that Cogburn died three days before and has him moved to her family plot. The film ends with Mattie standing over his grave and pondering how time catches up with everyone.