Jud Hendricks, foreman of the construction camp, is being blackmailed by Gypsy Joe, who knows of a dark page in the Hendricks' past. Hendricks and Tom Rasom are rivals for the favor of Helen, with Tom in the lead. The latter, an engineer, is about to take his train out when he finds Gypsy Joe hiding in a boxcar.
Learning that the driver of the Comet car has been disabled on the eve of the big race, Sinton, a gambler, bets heavily on its rival. But his plans go awry when Gordon, the owner of the Comet car, meets Naroche, a celebrated French driver, and engages him to pilot the racer.
Greggs, returning from abroad with a large consignment of precious stones, thwarts the first attempt of Gentleman Joe and his accomplice to rob him at his hotel, but they follow him aboard the train the when he is alone on the observation platform they attack him, and a struggle ensues in which Greggs is finally thrown to the ground from the speeding train. Helen, riding through the hills in an auto, comes upon him before Joe and his pal can alight from the train, Greggs gives her the diamonds and tells her to speed away and rush help back to take care of his injuries.
Helen, discharged by the superintendent without justification, comes to the rescue when a flat car, loaded with dynamite, is tearing to certain destruction down the grade, bearing the mischievous son of the superintendent.
Escaping after an early morning bank robbery, Gentleman Joe and his pal succeed in boarding a freight train headed toward Lone Point. Fearing rightly that a warning has been sent down the line, they secrete their loot in a box car, and, after noting its number, alight and seek cover until after the pursuit has cooled.
Helen, by a courageous leap from a motorcycle, reaches the burning boxcar in which the detectives are imprisoned and succeeds in applying the brakes in time to bring it to a stop and save them from almost certain death.
The new superintendent scoffs at the men's fear of Engine 3615, and declares that it must be put into service at once. Engineer Kent, a veteran of the road, refuses to take the throttle and is discharged. Dick Benton, a young engineer, is induced to take the engine out and "kill this talk of a hoodoo."
Stallings' plot to spoil the demonstration of Dick Benton's newly invented safety stop for trains seems certain of success when the locomotive is sent running wild down the tracks. Helen saves the day by climbing out on a wire stretching across the tracks and dropping to the speeding engine.
On a visit to the State Prison with Superintendent Melvin of the construction camp near Lone Point. Helen gains the friendship of Butler, a former telegrapher who had been wrongfully convicted on circumstantial evidence. Butler is soon to be released and Helen promises to aid him.
Whispering Smith, a railroad detective, is sent to Medicine Bend to suppress the looting of cars.
Following the wreck of the pay-car, it is rifled by a band of conspirators. All but one of the band later succeed in making their escape from the town by automobile, but this one is forced to jump aboard a freight train to which is attached the wreck train and huge crane.
Trent returns to the throttle too soon after his recovery from illness. On the following day Ruth, his daughter, and Hume plan to elope. Trent, with a new fireman, is suddenly taken with a weak spell and when the fireman becomes panic-stricken there is a scuffle which ends with the fireman thrown to the ground while the fast freight speeds on.