It's 1882 and the small town of Appaloosa in the New Mexico territory is being bullied by powerful rancher Randall Bragg (JEREMY IRONS) and his gang of thugs. When a sheriff and his deputies arrive on Bragg's ranch to arrest two of his men for murder and rape, Bragg shoots all three lawmen dead.
As a result, the elected officials and businessmen of Appaloosa, including store owner Phil Olson (TIMOTHY SPALL) hire gunmen Virgil Cole (ED HARRIS) and Everett Hitch (VIGGO MORTENSEN) to bring order back to their town. With a well-deserved reputation that precedes them, the two immediately enact their own laws and start with shooting and killing three of Bragg's men in town.
While Everett ends up seeing Katie (ARIADNA GIL), Virgil finds himself enamored with Allison "Ally" French (RENÉE ZELLWEGER) who's just arrived in Appaloosa, and it's not long before the two are building a house in town together. But work must come first, and after one of Bragg's hired hands, Joe Whittfield (GABRIEL MARANTZ), agrees to testify about witnessing his boss kill the three lawmen, Everett and Virgil arrest the rancher and then await the arrival of a traveling judge for the pending court case.
That draws in all sorts, including brotherly gunslingers Ring Shelton (LANCE HENRIKSEN) and Mackie Shelton (ADAM NELSON) who make Virgil wary, but don't shake his resolve. From that point on, and following Bragg's escape, the two steely lawmen do what they must to bring him back to justice.