Mattie Ross, a fourteen-year-old from a prosperous Arkansas family in in the 1870s, would in other circumstances be an insufferable brat. Right now, though, she's a young woman on a mission: her father has been shot and killed by a transient worker of dubious background that Frank Ross had hired to work on their farm. The killer, Tom Chaney, has fled across the border into Choctaw territory, and Mattie is determined to get him. To that end, she has found the "meanest" Federal Marshall, Reuben "Rooster" Cogburn, and offered him a hundred dollars to help her track down Chaney.
Cogburn is initially reluctant to bring a young girl with him into Indian Territory, and when Mattie has worn him down, a Texas Ranger LeBoeff, following Chaney, whom he knows as Chelmsford, for killings in Texas, reiterates the very sensible idea that Mattie should go home and let them track the fugitive.
That's not happening.
Mattie is almost as tough and smart as she thinks she is, and she, Cogburn, and LeBoeff learn not to underestimate each other.
Mattie is telling this story from her childhood many years later, an aging spinster bank owner who has even yet never seen any reason to compromise her standards or her beliefs. She can be naive, judgmental, narrow-minded, but she is always utterly sincere. Donna Tartt's reading is pitch-perfect.