After Bill Doolin is released from jail following his false arrest at the hands of crooked railroad detective Sam Swanson, he takes a train to his new ranching job in Cimarron County, Oklahoma. On the way, the notorious Dalton gang robs the train, and after gang member Red Buck inadvertently identifies Bill as a friend of the Daltons, Bill is forced to flee the wrath of the other train passengers. Just as Bill reaches the ranch, Swanson finds his abandoned luggage on the train, calls Bill into his office and immediately tries to beat another false confession out of him. Bill escapes and runs to the Daltons', who welcome him into their gang, a close band consisting of the Dalton brothers--leader Bob, Bitter Creek, Dynamite Dick, Will, Grat and Emmett--and Red. They plan a double robbery in which the brothers and Bill are to pilfer the Coffeyville bank while Red robs the Independence bank, after which they will all meet at Pat Roberts' ranch. Red's robbery succeeds but the Coffeyville raid results in a shootout, during which all of the gang members are killed except Bill, Bitter Creek and Dynamite Dick. The three outlaws rejoin Red, Will and Bitter Creek's Mexican girl friend Rose at Roberts' ranch while Swanson and Bill's friend, Marshal John Sutton, begin to track them down. Bill impresses Roberts' daughter Carrie when he convinces the remaining members to ignore Red's bloodthirsty revenge plans and instead kill only when absolutely necessary. Bill is immediately named the new head of the gang, and is dubbed "The Cimarron Kid." Swanson and Sutton then arrive at the ranch, and although the gang hides in the stables, Sutton deduces their location and loudly announces that he knows Bill was coerced into a life of crime and that he would be willing to cut a deal. After he leaves, Carrie reveals her attraction to Bill by denouncing him for continuing to steal, and he informs her that he just wants to acquire enough money to move to Argentina and start a new life as a cattle rancher. Over the next few months, the gang pulls off a series of brilliant bank robberies across the West, and Swanson increases his efforts to capture them. They are almost caught in Boonesville, when Bill spots Carrie visiting the town and stops to talk to her. Although he escapes, Swanson hears of the incident and arranges a posse. The gang eludes the posse, only to fall prey to a setup in the town of Columbia when hotel owner Lola Plummer promises them protection but then turns them in. Just when they appear to be trapped, Rose helps them to escape once again, although Red is killed, Bill wounded and Dynamite disappears into a river. They retreat to their hideout in the hills, where Bill convalesces and Dynamite eventually returns, unharmed. He brings with him his brother-in-law, George Weber, a railroad employee who lets them in on his plot to steal gold bricks being shipped to Denver. Later, when Carrie comes to visit Bill, he promises her the gold heist will be his last job, and that he will come for her. Bill, Dynamite and Weber steal the gold bricks and toss them off the train one by one while Bitter Creek and Will wait at each stop to pick them up. Rose realizes that Dynamite and Weber have set them up when she sees Bitter Creek and Will get shot by Swanson's men as they try to retrieve the gold. She sends a telegram to Weber, signed with Swanson's name, which informs him of the murders. When Weber receives the telegram, Bill, sensing something is amiss, reads it and wounds both Weber and Dynamite. A few miles later, Bill jumps off the train and runs to the Roberts'. Pat and Carrie put him to bed but then turn him in to Sutton, knowing arrest is his only chance for redemption. Bill initially refuses to speak to Carrie, but after Pat explains to him that once Bill is released from jail, he will sign his ranch over to Bill and Carrie, Bill kisses Carrie goodbye and promises to return.