Tom Horn (Steve McQueen) served in the military where he participated in capturing Geronimo.
He gains fame for his ability to track a man. He is hired by John Coble (Richard Farnsworth) to stop cattle rustlers. As the bodies start to pile up, the cattle men realize Horn is bringing bad publicity. Local Marshall Joe Belle wants to build his career by bringing in the notorious gunslinger. When a local teenager is shot tending sheep, they decide to pin it on Horn.
Belle lures Horn into a barroom conversation about the murder, with hidden witnesses. While Horn never admits to the murder, his cryptic statements are taken as a confession. Horn is arrested for murder. He breaks out of jail, but is quickly captured. He is hanged for the murder.
Thanks Joseph C, who also notes:
Tom Horn was a real life Old West figure, who served as a hired gun. This movie presents the idea that he was hanged for a murder he did not commit, not any of the ones he did commit. Teenage Willie Nickell was shot tending sheep. There was no CSI then, and no modern forensic technique, so we will never know for sure. However, it is most likely that Tom Horn was really guilty. Horn had threatened to kill Willie's father, and Willie was wearing his father's coat at the time. From a distance, he could be mistaken for his father. An expert marksman shot Willie from a long distance. There was no one else who would have wanted to shoot Willie.