Terrorizing 1866 Indiana, the Reno brothers gang uses the town of Seymour as a safe haven, paying off three crooked town officials. Sent in to clean up the gang is Peterson Detective Agency operative James Barlow, who poses as an outlaw to gain the confidence of the officials and the thick-headed brothers. Complicating matters are Barlow's feelings for the Reno sister, Laura, who reluctantly keeps house for the boys out of family loyalty. Events heat up and rage surfaces as Barlow sets up the gang in a dawn train robbery.

In 1866, the Reno Brothers gang--leader Frank, Simeon, John and Bill Reno--ride into North Vernon, Indiana, to rob the bank. Before they enter the building, however, lawmen lying in wait open fire on them, killing Bill. The rest of the gang flees the pursuing posse, and later, at their sister Laura's home in Seymour, Indiana, the surviving brothers debate how the authorities came to know of their plan. They first suspect their brother Clint, a law-abiding farmer, of the double-cross, but finally conclude that Murphy, a new bartender in town, must have passed on the information after a drunken Sim unwittingly revealed their plot to him. In revenge, the Renos, who operate in collusion with Seymour's constable, Brant, prosecuting attorney Lattimore and Judge Hawkins, tie Murphy to a barn post and set the barn on fire. Outraged by his agent's murder, Amos Peterson of the Peterson Detective Agency, hires the sharpshooting Jim Barlow, a former Confederate spy, to infiltrate the gang. To that end, Jim and Monk Claxton, another agent, pretend to rob a train and then, while posing as an artist and his assistant, respectively, move in with an old Seymour farmer named Noah Uhalt. Jim, a notorious ladies' man, immediately catches Laura's eye, but when she tries to chat with him, two of the Renos' henchmen pick a fight. With Monk's help, Jim knocks out the outlaws and claims their guns. Meanwhile, Brant, Lattimore and Hawkins, having read newspaper accounts about the train robbery, suspect that the Renos were behind it and accuse them of withholding their share of the loot. After the Renos angrily deny any involvement, Hawkins receives word that a $100 bill from the holdup has been located and Jim is identified by store owner Fisher as the bill's source. Later, when Jim shows up at Laura's house to return the confiscated guns, Frank orders him away, over the protests of Laura, who has grown weary of her brothers' criminal ways. Jim and Monk then are arrested, and during his interrogation, Jim admits his guilt and mentions to Hawkins that he has a "contact man" at the express office and, if released, will cut Hawkins and his cronies in on any future robbery take. As hoped, the officials jump at Jim's bribe and persuade the Renos to join forces with him. During their first holdup in a Missouri express office, Jim is compelled to shoot and wound a teller who is going for his gun. Although the robbery is a success, Jim later complains about the small take and openly challenges Frank's leadership. While Jim is with the Renos, Laura spies on him and tearfully calls him a lying thief, accusations he cannot deny. Later, Jim meets secretly with Peterson and Sheriff Moseley in North Vernon to arrange a heist that will ultimately trap the Renos and the politicians. As part of the plan, Jim shows Hawkins, Lattimore and Brant a coded telegram he claims to have received from his express office contact, which details an upcoming $100,000 shipment of money. Despite their dislike of Jim, the Renos agree to participate in the train holdup, under Jim's direction. After the unsuspecting outlaws stop the train and demand the strongbox, lawmen surround them, and a fierce gunfight ensues. Monk is killed in the confrontation and Frank wounded, but all the Reno brothers are arrested, and Hawkins, Brant and Lattimore are exposed. That night, however, Fisher and his helper, Deadrick, incite the townspeople to storm the North Vernon jail and, while masked, lynch the Renos. Laura, meanwhile, sneaks to Noah's ranch and seeks comfort from Jim, whose mission she now understands and accepts. Clint, who has overheard the vigilantes' plan, then rides up and alerts Jim, but Jim is unable to prevent the hangings. Jim does identify Fisher and Deadrick as the mob leaders, but they refuse to implicate anyone. Later, Jim apologizes to Laura for her brothers' fate, but she reassures him that she still loves him and looks forward to sharing her life with him.