As Mattie's tale begins, Chaney is employed on the Ross’ family farm in west central Arkansas, near the town of Dardanelle in Yell County. Chaney isn't much use as a farmhand and Mattie has only scorn for him, referring to him as "trash." She says her father, a good, kind man, only hired him out of pity. One day, Frank Ross and Chaney go to Fort Smith to buy some horses. Ross takes $150 with him to pay for the horses, along with two gold pieces he always carried. When Ross tries to intervene in a barroom confrontation, Chaney kills him, robs the body, and flees into Indian Territory (present day Oklahoma) on his horse.
Hearing that Chaney has joined an outlaw gang led by the infamous "Lucky" Ned Pepper, most of the local marshals refuse to give chase. Mattie means to track down the killer, and upon arriving at Fort Smith she looks for the toughest deputy Marshal in the district. That man turns out to be Reuben J. “Rooster” Cogburn, and although he is an aging, one-eyed, overweight, trigger-happy slob who never seems to miss a drink of whiskey, he also has “grit.” Mattie decides she's found her man.
Playing on Cogburn's need for whiskey money, Mattie finally persuades him to take on the job, insisting that, as part of the bargain, she must go along. During the negotiations a Texas Ranger named La Boeuf appears. He, too, is tracking Chaney for killing a senator in Texas, and is out for glory, and a big cash reward. Cogburn and La Boeuf don't much like each other, but, after some haggling, they agree to join forces in the hunt. The two men try hard to leave Mattie behind, but she proves more tenacious and resourceful than they'd expected and eventually she becomes an accepted member of the posse.
Together, but with very different motivations, the three ride into the wilderness to confront Ned Pepper's gang. Along the way, they begin to appreciate each other a little more.
True Grit is an adventure story told in a plain style - Mattie's style. Portis frames the narrative in the tone and perspective of a Puritanical (and, at first glance, rather disagreeable) Arkansas frontier woman. In keeping with Mattie's worldview, the story is replete with Biblical motifs, especially the “eye for an eye” style of justice. Portis also blends a good deal of deadpan humour with the book's serious themes of murder and retribution.
An important element of the story is the comradeship that develops between Mattie and Cogburn despite their very different characters. Portis contrasts Mattie’s strict, abstemious nature with Rooster’s abundant indulgence in both alcohol and profanity. However, the two do have one trait in common, and this allows them to develop a strong mutual respect: they both have "true grit." Other characters - notably LaBoeuf and Ned Pepper - also, for good or ill, possess this trait. With this gallery of (mostly) rogues, Portis illustrates the many faces of courage, and the various uses to which courage can be put.
True Grit is arguably a story of transitions, particularly the parallel transitions of Mattie. First, and most obvious, are the physical changes: the loss of her forearm from snakebite, her development from girlhood into womanhood, and, on a symbolic level, the death of her horse. Second is the implied emotional damage Mattie suffers; after she avenges her father's murder, she never marries, instead becoming a formidable old maid working in a modern-day fortress - a bank. Third, Mattie's adventures challenge one of her deepest conceits; an overreliance, amounting to self-righteousness, on words and precepts. She begins by assuming that the answers for every problem can be found in the Bible, the Law, and the Protestant work ethic, and she tries to handle every situation by aggressively quoting both the Scriptures and the wise sayings of her attorney, J. Noble Daggett (whom she "draws like a gun”). She later discovers that some situations cannot be managed so easily; in the wilderness, her homilies bounce off unpredictable men like Chaney, Cogburn, La Boeuf, and Ned Pepper.
In this view, Rooster and La Boeuf also experience changes. Rooster, once a henpecked storekeeper in Illinois, is, when Mattie first sees him, a hardened lawman whose "family" consists of a roommate, a horse, and a cat. Getting to know Mattie, and learning to respect her courage, gives him a new allegiance. The selfish drunkard Mattie meets in Fort Smith becomes her hero, first facing four men alone in a shootout, then rescuing her from a snakepit. Afterwards, in search of a doctor, he rides her horse until it drops dead, then carries Mattie in his arms, and finally steals a wagon and team. La Boeuf, too, moves from obsession with the money and glory of capturing Chaney, and the pride of being a Texas Ranger, to a measure of humility and self-sacrifice. Although he's suffering from a head injury, he assists in Mattie's rescue.
It is possible, however, that Mattie, Rooster, and La Boeuf don't actually change that much in the course of the novel; they simply improve upon closer acquaintance. Mattie's later life demonstrates her unquenched high-mindedness, together with a remarkable steadiness of character; once Rooster returns her to civilization, she resumes her old responsibilities. Rooster, too, remains true to his lights, and ends up in a wild west show. The purpose of the novel, then, may be to show courageous people remaining steadfast in the face of their worst fears (as Mattie does when she falls into a pit full of snakes and human bones), not to show them learning to amend their characters through hardship.
A film sequel, Rooster Cogburn, was made in 1975, with John Wayne reprising his role, and Katharine Hepburn as an elderly spinster, Eula Goodnight, who teams up with him. A made-for-television sequel, entitled True Grit: A Further Adventure, was made 1978, starring Warren Oates and Lisa Pelikan, and featured the further adventures of Rooster Cogburn and Mattie Ross.