In 1876, Johnny Jameson [Dan Dailey] is the sole passenger on the inaugural run of the Tomahawk and Western Railroad’s narrow-gauge line slicing through the Colorado Rockies. The train, driven by the line’s lone locomotive Emma Sweeny, hums along as a cautious conductor warns that some interests—like stagecoach operators—would cheer to see the franchise fail. Soon a threat materializes: Dakota [Rory Calhoun], Trancas [Charles Kemper] and Gila move to drop a massive boulder straight into the rails, a sabotage meant to derail the project and undercut the town’s future.
The emergency is averted when the engineer Terence Sweeny [Walter Brennan] swings the locomotive to a stop just in time, and the crew hops off to move the rock clear. Johnny, shaken but determined, choices a path toward town, hitching a ride with Dakota’s accomplices—Trancas and Gila—toward Epitaph. At the sheriff’s office, Johnny tries to report the delay to deputy Chuckity Jones [Charles Kemper], only to be knocked out by Trancas. Nearby, U.S. Marshal Dodge [Will Wright] prepares to greet the train with his granddaughter, Kit Dodge Jr. [Anne Baxter], a formidable and capable young woman who carries knives as readily as she sways from a horse. As they depart toward the depot, the pair are surprised by Trancas and Gila; a brief exchange ends with Dodge firing at Trancas and wounding him, while Gila escapes, and Johnny begins to recover his bearings.
Kit, newly deputized as a U.S. Marshal under her grandfather, is assigned to escort the delayed train, joined by Pawnee [Chief Yowlachie], a loyal indomitable companion. Colonel Dawson orders Dakota to join the security detail and dispatches an Indian scout, Black Wolf, to stir up tensions with the local Arapahos. The gang’s plan grows more treacherous as they plot to sabotage the engine during a night stop.
A new complication arises: there is no track laid for nearly forty miles. Bishop, a shrewd railway entrepreneur, explains that rails were lost at sea en route from England, and that the Emma Sweeny must somehow reach Tomahawk by a strict deadline to fulfill the franchise. The requirement also demands at least one paying passenger, which means Kit will be responsible for Johnny Behind-the-Deuces. The plan forces the engine to move without a passenger car, hauled by a team of mules and flanked by wagons. Joining the caravan are Long Time [Victor Sen Yung], a Chinese laundryman bringing delayed laundry for Tomahawk, along with Madame Adelaide [Connie Gilchrist] and her troupe—Annie, Ruby, Clara [Marilyn Monroe], and Julie—who bring a bit of entertainment, accompanied by a pianola player.
Dawson’s gang makes a late-night move: Bat, Charley and Fargo infiltrate the camp, posing as telegraph men to repair lines cut by the Arapahos. Kit, wary but practical, allows them to bunk for the night. Johnny persuades Madame Adelaide and the dancers to perform for the weary travelers, and Kit’s hard exterior begins to soften as she watches him interact with the troupe.
The quiet of the camp shatters when Bat and Charley depart and Fargo plants dynamite beneath the engine. Johnny, sleeping beside the train, smells the fuse and awakens the sleepers. Kit uses a shot to sever the fuse and disables Fargo, who falls to the ground, a violent end delivered by Dakota, who arrives to finish the job.
Not long after, Bat and Charley reappear at a second night stop, placing charges under a trestle. Johnny, Kit and Pawnee scout ahead and encounter danger on the bridge. The bandits attack; the trio fights back, and Dakota’s deft hands reveal his connection to Crooked Knife, an Arapaho chief Johnny once knew from a traveling western show. The war party is driven away, yet Johnny decides to seek peace with Crooked Knife. He learns Long Time is carrying a cache of fireworks and crafts a plan to win the trust of the chief by staging a signal that he is “big medicine.”
When Crooked Knife agrees to safe passage, Johnny signals Kit and Dakota to release the rest of the fireworks on a nearby hill, impressing the Arapaho with a display of light and color. With the bridge now out, Kit plans a daring solution: dismantle the locomotive and haul it over the mountain in pieces. Dawson, wary of betrayal, discovers Black Wolf and shoots him before mobilizing his men for a final confrontation.
The Emma Sweeny is stripped to its essential components and hauled over the mountain by mule teams, then reassembled on the track. Kit discovers that someone sabotaged the water tower intended to fill the tender, but Dakota’s miscalculation means the tender remains magically full already. As Dakota realizes the plan has been uncovered, he leaps aboard, clubs Johnny, and forces the fireman to move the locomotive ahead, leaving Kit behind in the cab. Dakota fires a shot at Kit, but finds himself low on ammunition and, with a desperate throw, drops his pistol at her feet. Kit catches it midair, and a swift knife throw from Johnny’s ally hi-jacks the trajectory; Dakota tumbles to his death in a ravine.
Dawson and his gang ambush the train again, but their assault sabotages only the boiler rather than the whole operation. The Emma Sweeny loses steam, yet a town posse led by Marshal Dodge appears from Tomahawk, aided by the Arapahos. The showdown concludes with Pawnee tackling Dawson and ending him with a precise tomahawk throw, bringing the threat to a close.
With the danger behind them and the train near Tomahawk, Johnny works to persuade the mayor to extend the town limits, just enough to secure the franchise’s requirements. Seconds before the deadline, he succeeds. A tender moment follows as Kit confesses her love for Johnny, only to learn that his wandering life would keep them apart. He insists that he cannot abandon the road he travels.
Yet time has another plan. Several years later, Johnny is married to Kit and serving as Tomahawk’s train conductor. As the train pulls away, he limps after it, waving to Kit and their five young daughters—each named in homage to Madame Adelaide’s dancers—as the landscape of their shared life continues to roll forward.
