It's the 1870s and Mattie Ross (HAILEE STEINFELD) is a 14-year-old Arkansas girl who's mature beyond both her age and the era in which she lives. She puts her determination to use in wanting to hire someone to find and capture Tom Chaney (JOSH BROLIN), a handyman who's just killed her father.
No one takes her seriously, including Texas marshal La Boeuf (MATT DAMON) who's been tracking Chaney for a long time and over great distances following the man having also murdered a Texas senator. He believes that Chaney is traveling somewhere in hostile Indian territory and may have met up with his cohort, Lucky Ned Pepper (BARRY PEPPER).
La Boeuf wants nothing of her offer to hire someone to find her father's killer and bring him to justice, a sentiment shared by Rooster Cogburn (JEFF BRIDGES), a federal marshal with a gritty reputation of getting the job done but usually with a high body count. Despite his demeanor, Mattie is sure Cogburn is the man for the job and thus hires him with the condition that she travel along with him.
He agrees, and leaves the next morning, not with her by his side, but La Boeuf who wants to take Chaney back to Texas for a handsome reward. Mattie will have no part of that or being left behind. Accordingly, she rides out and finds both, and then continues with them on their manhunt. From that point on, they travel across potentially dangerous lands, following leads they hope will take them to Chaney.