In 1878 New Mexico Territory, John Tunstall, a British rancher, hires young wayward gunmen to live with him and work on his ranch, offering them an alternative to a life of petty crime. The group, who quickly come to view Tunstall as a mentor and father figure, includes the brash and impulsive William H. Bonney, nicknamed Billy the Kid, the thoughtful and articulate Josiah "Doc" Scurlock, and the stoic Cherokee half-breed, José Chavez y Chavez, among others. When Tunstall is gunned down by the corrupt Lawrence Murphy, a wealthy ranch owner who controls the town with his illegal enterprises and has the local law in his pocket, the young men are left devastated and hungry for justice.

Swearing to avenge Tunstall’s murder, the group is deputized by Alex McSween, a lawyer and ally of Tunstall, to serve arrest warrants to the corrupt individuals responsible for his friend’s death. The young deputies are now pitted against Murphy and his hired guns, including the cutthroat Murphy loyalist, Buckshot Roberts, who will stop at nothing to maintain their stranglehold on the territory. As the situation spirals out of control, leading to a series of violent confrontations, the group earns the moniker "The Regulators," and become entangled in what will become known as the Lincoln County War, a bloody power struggle that will test their resolve and their loyalty to one another.