In 1950, esteemed attorney Charles Phalen is unexpectedly visited by the elderly Brushy Bill Roberts, who claims he has a long-promised pardon from the governor of the New Mexico Territory. Brushy Bill's assertion that he is actually William H. Bonney, famously known as Billy The Kid, is met with skepticism. To prove his identity, Brushy Bill begins recounting his notorious tale which traces back to 1879, when Billy teamed up with "Arkansas" Dave Rudabaugh and the infamous Pat Garrett to form a new gang.
As the legendary outlaw navigates the perilous landscape of post-Lincoln County War New Mexico, Governor Lew Wallace issues warrants for all involved. Doc Scurlock, once Billy’s confidant and now a schoolteacher in New York, finds himself imprisoned alongside fellow Regulator Jose Chavez y Chavez and their former enemies.
Now on the run and labeled a most wanted man, Billy attempts to confront Governor Wallace, only to discover he has been tricked into arrest with no chance to testify. He manages a daring escape and reunites with Dave and Garrett, who pose as a lynch mob to rescue Doc and Chavez, convincing them to join their dangerous escapades along the "Mexican Blackbird" trail to Mexico.
In desperate need of reinforcements, Billy’s gang reluctantly accepts new members — the farmer Hendry William French and the young Yankee Tom O'Folliard. Meanwhile, Garrett opts to stay behind, focusing instead on opening a boarding house. Seeking reparation, Billy confronts former ally John Chisum, resulting in violent conflict that ends with two of Chisum’s men dead.
Furious over Billy’s actions, Chisum allies with Wallace to offer Garrett a lucrative proposition: become Lincoln County sheriff and hunt down Bonney for a $1000 reward. A posse is formed, with a journalist recruited to document their relentless pursuit. In a retaliatory act, Billy leaves a taunting message for Garrett, and tension escalates when Rudabaugh unwittingly disturbs an Apache burial site, leading to a chaotic knife fight with Chavez.
Upon reaching the town of White Oaks, the gang spends the night at Jane Greathouse's bordello, triggering a flood of memories from their past. However, their respite is short-lived as a lynch mob gathers outside. Deputy Carlyle, facing pressure, offers the gang leniency in exchange for Chavez. Billy cleverly dresses Carlyle in Chavez’s clothes, pushing him outside where he meets a grim fate at the hands of the mob.
As Garrett leads the pursuit to the bordello and sets it ablaze, Jane takes a bold stand, stripping bare to shame the townsfolk before disappearing into the night. With Garrett’s posse tightening their grip, Tom's tragic death serves as a stark reminder of the Mexican Blackbird's intent to hold the gang together. As they draw closer to the end, Doc bravely attempts to escape but is brutally shot down by one of Garrett's men, sacrificing himself to secure his friends' freedom, leaving Billy captured and Chavez gravely injured.
The authorities soon deliver a death sentence for Billy, but before his execution, Jane pays a fateful visit, gifting him a pistol that will play a crucial role in his escape from Lincoln's outhouse. After dispatching two guards, Billy makes a daring getaway to Fort Sumner, reuniting with his gang along the path.
However, their reunion is fleeting; Dave has already fled to Mexico, and Chavez succumbs to his wounds. On that fateful night, an unarmed Billy confronts Garrett, imploring him to allow him to escape to Mexico on the condition he informs authorities of his own death. Garrett, unmoved, readies himself for a fatal confrontation.
The following morning, what appears to be Billy's burial takes place, but Garrett's horse mysteriously vanishes, hinting at a deeper conspiracy. In 1950, Brushy Bill shares his remarkable story, convincing Phalen of his identity as the legendary Billy the Kid. A poignant conclusion reveals Dave's grim fate in Mexico, serving as a stark warning to outlaws of the era, while Garrett’s book flounders, leading to his untimely death in 1908.
As Brushy Bill meets with the New Mexico governor, despite the testimonies from those who knew the Kid, he is dismissed and passes away shortly after, leaving behind a haunting mystery about Billy’s true legacy.
