When Jim Keaton learns that his brother Billy has died and has left him an inheritance, Jim leaves his home in Del Rio, Texas, a sleepy border town along the Rio Grande River, and travels to Denver. He hears about his brother's dealings with the diamond fields and sets out to investigate.
Jim finds himself settling into the corner where Colorado, Utah, and Wyoming meet. While looking for the diamond field, he runs into hardheaded Ed Cole, a man set in his ways who uses public ground for sheep grazing. Befriending Cole places Jim in the middle of a nasty range dispute that could become deadly at any moment.
Suspicion arises when Jim recognizes Tom Horn, a former Indian scout but now a range detective with a reputation for eliminating rustlers without a trial. Horn is working on Cold Spring Mountain masquerading as a man named James Hicks. When men start dying, blame is placed on Horn. Only Jim knows the truth, and he's not sure he can or wants to handle that burden.