Cowboy Ross McEwen arrives in town. He asks the banker for a loan of $2000. When the banker asks about securing a loan that large, McEwen shows him his six-gun collateral. The banker hands over the money in exchange for an I.O.U., signed "Jefferson Davis". McEwen rides out of town and catches a train, but not before being bitten by a rattler. On the train, a nurse, Miss Hollister, tends to his wound. A posse searches the train, but McEwen manages to escape notice. However a mysterious Mexican has taken note of the cowboy, and that loudmouthed brat is still nosing around. Who will be the first to claim the reward for the robber's capture?

While the citizens of the New Mexico town of Santa Maria are welcoming their new U.S. Marshal, Pat Garrett, Ross McEwen, posing as Jefferson Davis, is holding up the local bank. After he gives banker Frenger an I.O.U. for the $2,000 he has demanded, Ross takes him out of town and leaves him shoeless and horseless. When Frenger eventually gets back to town, he offers a reward of $3,000 for Ross, dead or alive. Hoping to board a passing train, Ross, meanwhile, turns his horse loose and hides his saddle in some brush, where he is bitten by a rattlesnake. After making a tourniquet and sucking out the venom, Ross runs after the train and boards with the help of Mexican gambler Monte Marquez. On board, Fay Hollister, a railroad nurse from the East heading to a hospital in Alamogordo, tends to his wound. When a posse discovers Ross's saddle near the railroad tracks, Garrett wonders if he may have escaped by train. Ross's escape is interrupted when news comes that the rail tracks have been washed out and all the passengers are forced to leave the train at Albuquerque, where a conductor advises them that the next train out may not arrive for a week but that a mail hack will be able to take a few passengers the next morning. Garrett, Ross, Fay and Monte decide to take the two-day mail trip, and make their first stop at a way-station near Inscription Rock. Monte translates the inscription made by conquistadores, as they journeyed by the rock: "Pasó por aquí"--"he passed this way." After their meal at the station, "wanted" posters for the Santa Maria bank robber arrive and Fay recalls that Ross boarded the train near there. Back on the train, as they are nearing Alamogordo, Fay tells Ross that if he is in any trouble, she would like to help. Ross writes a note to his father, which he sends on with the money he stole, then when the train reaches Alamogordo, he disembarks with Fay and they embrace. Monte, too, leaves the train and invites Ross to the Long Horn Saloon, which he co-owns with his cousins. Monte, who also suspects that Ross is the wanted man, helps him by introducing him to cattleman Burnett. Burnett hires him as top hand, and Ross continues to court Fay, who, aware that there is something in his past, urges him to clear himself with the law. Garrett and his deputy, Clint Waters, meanwhile, are on Ross's trail and when they reach Alamogordo, the sheriff tells them about the new man at Burnett's ranch. After Ross asks Monte to send some of his new earnings to the bank in Santa Maria and to say that it is from Jefferson Davis, Garrett comes to Monte's saloon. Although Ross is standing only a few feet away, Monte does not turn him in to Garrett. Ross then tells Fay that he must leave for a while, and presents her with a ring before riding away. Fay follows him and although he tells her about the robbery and urges her to go back, she stays with him. Garrett and a posse, as well as two bounty hunters, pursue the couple throughout New Mexico. Finally, when Ross will not give himself up, Fay leaves him and is captured by Garrett. To cross the White Sands desert, Ross exchanges his horse for a steer and, on the other side, attempts to acquire a horse at a small ranch. However, when Ross discovers that the Mexican rancher, Florencio, his wife and two sons all have diphtheria, he stays to help them. After trying a home remedy, Ross realizes that they need professional help, which is a two day ride away. In the hope of attracting someone's attention, Ross lights a fire, which is seen by Garrett and Clint. They find Ross in bad shape, so Garrett sends Clint to Alamogordo for a doctor while he stays to work alongside Ross. When Clint returns with a doctor, as well as Fay and Monte, Monte, who is Florencio's nephew, still does not identify Ross to Garrett and introduces Ross to Fay as if they had never met. Later, with Florencio and family recovering, Garrett rides off, leaving the way clear for Ross to escape. After Monte tells him that he will meet him with supplies at Inscription Rock, Ross takes off, but Garrett who has been waiting for Ross to leave, reveals that he knows who he is and talks him into giving himself up, promising to speak on his behalf. Before going with Garrett, Ross rides to Inscription Rock where he tells Fay that he is giving himself up but that they will be together soon. As a tribute to his friend's good deed, Monte tells Fay that Ross's name should be inscribed on the rock: "Ross McEwen, un caballero valiente, passed this way."