In the year 1950, attorney Charles Phalen receives an intriguing phone call from the elderly "Brushy Bill" Roberts, who has a fascinating tale to tell. Brushy claims he is seeking a pardon that was promised to him 70 years ago by the governor of the New Mexico Territory. Initially skeptical of Brushy’s assertion that he is, in fact, William H. Bonney, famously known as "Billy The Kid," whom many believe was killed in 1881, Phalen probes deeper and asks Brushy for any proof to substantiate his remarkable claim.
Brushy’s narrative transports us back to 1879, during a tumultuous period when the notorious outlaw, Billy the Kid, emerges with a new crew, joined by "Arkansas" Dave Rudabaugh and Pat Garrett. Following the chaotic aftermath of the Lincoln County War, the situation escalates as New Mexico's Governor Lew Wallace issues warrants for the arrest of every participant. Among those caught in the crossfire is Billy's old friend Doc Scurlock, who has since become a schoolteacher in New York. He finds himself imprisoned, alongside fellow Regulator Jose Chavez y Chavez, as well as their longstanding adversaries.
Amidst the chaos, Billy, now the most wanted man in the region, cleverly seeks an audience with Governor Wallace, who tantalizingly offers a pardon in exchange for Billy's testimony against the Dolan-Murphy faction. However, Billy soon realizes he has been deceived, finding himself arrested with no opportunity to testify in his defense. In a cunning escape, he reassembles with Dave and Garrett, masquerading as a lynch mob to liberate Doc and Chavez. Reluctantly, they agree to join him on a daring expedition known as the "Mexican Blackbird" trail towards Mexico. In dire need of reinforcements, the gang incorporates farmer Hendry William French and a youthful deserter, Tom O'Folliard, while Garrett opts to remain back to manage a boarding house.
In a gripping turn of events, Billy confronts former ally John Chisum over a $500 debt, which tragically results in the demise of two of Chisum's men. Infuriated, Chisum allies with Wallace, offering Garrett the position of Lincoln County sheriff and a hefty $1000 to track down Billy Bonney. As Garrett forms a posse, he brings along a journalist to document the relentless pursuit. In a series of playful taunts, Billy mocks Garrett, and tensions mount when Rudabaugh inadvertently disturbs an Apache burial ground, leading to an intense knife fight with the Mexican-Indian Chavez.
The gang's journey leads them to the town of White Oaks, where they spend a fateful night at a bordello run by none other than Billy's former companion, Jane Greathouse. The atmosphere grows thicker as a lynch mob descends upon them, and in a gruesome twist, Deputy Carlyle offers them a chance to escape, demanding Chavez's surrender. In a bold move, Billy dresses the deputy in Chavez's clothing and sends him outside, where he meets a fatal end at the hands of the mob. As the gang's fortunes decline, Garrett relentlessly hunts them down, ultimately setting the bordello ablaze while Jane exploits the moment to publicly humiliate the townsfolk before making her escape.
With the relentless pursuit pressing down, tragedy strikes when Tom is gunned down by Garrett. In a moment of vulnerability, Billy the Kid reveals that the much-touted Mexican Blackbird was merely a ruse to keep his crew intact. Amidst growing despair, Doc attempts to leave their perilous odyssey behind but is mortally wounded by Garrett's men, sacrificing himself to allow the others to escape. Although Chavez is hurt, Billy is ultimately captured and dragged back to Lincoln, where he faces a death sentence. A chance meeting with Jane in prison leads her to smuggle a pistol into the outhouse, which Billy ingeniously uses to eliminate two guards during his daring escape towards Fort Sumner.
Reuniting with his gang, he learns of Dave’s flight to Mexico, while a dying Chavez seeks his own fate away from the group. In a tense standoff that night, an unarmed Billy confronts Garrett, pleading for a chance to flee to Mexico, assuring Garrett that he would allow him to assert he had killed him. Skeptical of Billy's intentions, Garrett vehemently refuses and prepares to shoot him in the back instead. The following morning sees a burial held for Billy the Kid, yet unbeknownst to all, Garrett’s horse is taken by an unseen figure, sowing seeds of intrigue.
As Brushy Bill brings his story to a close in 1950, Charles Phalen is left questioning the authenticity of Brushy’s claims, pondering whether he truly is the legendary Billy the Kid. The epilogue unravels unsavory fates for others involved: Dave is beheaded in Mexico as a grim warning to fellow outlaws; Garrett's published account fails to gain traction, ultimately leading to his own demise in 1908; and while Brushy Bill meets with the governor of New Mexico with reasserted claims of Bonney’s legacy, he is discredited, dying shortly thereafter. The truth of whether Billy the Kid really lived on remains shrouded in mystery, leaving room for speculation and imagination.
