In 1881, a young and impressionable Robert "Bob" Ford sets out to find the notorious Jesse James, who is planning a train heist in Blue Cut, Missouri. With the intention of joining Jesse's gang, Bob seeks the help of his older brother Charley, who is already an established member. The anticipated train is only carrying a small amount of treasure, leading to the disheartened departure of Frank James from the gang. Jesse makes his way back to Kansas City, Missouri, accompanied by the Fords, Dick Liddil, and Jesse's cousin, Wood Hite. Although Jesse sends Charley, Wood, and Dick on their way, he insists that Bob stay behind to assist him with some moving tasks.
During this time, Bob becomes increasingly infatuated with Jesse, but his admiration begins to wane when he temporarily lives at the farmhouse of his widowed sister, Martha Bolton, where he reunites with Charley and his newfound acquaintances. Tension builds when Liddil confides in Bob that he is plotting with fellow gang member Jim Cummins to betray Jesse for a hefty bounty. Meanwhile, Jesse pays a visit to another gang member, Ed Miller, who inadvertently reveals Cummins' plan. In a fit of rage, Jesse murders Miller before teaming up with Liddil to track down Cummins. Failing to find him, Jesse brutally attacks Albert Ford, a young relative of Bob and Charley.
The situation escalates when Liddil and Wood’s affair with Hite's stepmother is discovered. In a fit of jealousy, Wood confronts Liddil at the Bolton's and is shot and killed by Bob, who acts instinctively to protect Liddil. They attempt to hide Wood's body in the woods to keep Jesse unaware. Soon after, Jesse arrives for dinner at the Boltons, where the Fords deny having any knowledge of Liddil's whereabouts. Feeling mocked by Jesse's taunts about his blind admiration, Bob's feelings sour into resentment, particularly after he learns about the fate of his cousin.
As Jesse and Charley make their way to St. Joseph, Bob walks into the office of Kansas City Police Commissioner Henry Craig, professing his knowledge of Jesse’s location. To prove his loyalty, Bob assists in the capture of Dick Liddil, whose confession leads to a deal brokered with Governor Thomas T. Crittenden. He is given a mere ten days to capture or kill Jesse for a promised bounty and a pardon regarding Hite’s murder.
Eventually, Charley convinces Jesse to induct Bob into the gang. They play cousins while staying with Jesse's family, but Jesse's suspicions grow, preventing the brothers from being alone together. Yet, as days pass without incident, Jesse gifts Bob a polished, nickel-plated gun as a gesture of goodwill. On April 3, 1882, as Jesse prepares for the robbery, he discovers Liddil's arrest in a newspaper, prompting him to clean a dust-covered picture instead of being alert. Seizing the moment, Bob shoots Jesse in the back of the head with the gun Jesse had given him, and he and Charley flee. They promptly send a telegram to the governor announcing Jesse’s demise to claim the promised $10,000, though they receive a mere $500 each.
In an attempt to profit from Jesse's death, the Fords embark on a theatrical tour in Manhattan, reenacting the shocking murder. However, public sentiment shifts as audiences begin to call Jesse a legend and label Bob a "coward." The performances come to an abrupt halt when Bob's frustration boils over, resulting in him assaulting an audience member who mocks him. Wracked by guilt, Charley writes countless letters to Zee James seeking her forgiveness, though he never follows through with mailing them. His inner turmoil leads him to commit suicide in May 1884. Bob, trying to forge ahead with life, is haunted by his past transgressions. On June 8, 1892, his life is cut short when he is shot dead by Edward O'Kelley in his saloon in Creede, Colorado. O'Kelley receives a life sentence but finds himself pardoned a decade later in 1902.
