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J. G. "Pepper" Martin, the section boss of the Pacific Short Line railroad, is searching for the thieves who have been robbing ore shipments belonging to the Great Western Smelter. Without the smelter's business, the railroad will go bankrupt, and so, when Barker, the short line's claims agent suggests that he investigate Agent Jack Hathaway, who has only recently joined the company, Pepper agrees. Jack works at Smelter Junction, and that afternoon, is visited by Pepper's long-absent daughter Helen, with whom he has been friends since their childhood, and her dog Rex. Barker is displeased to see Helen interested in Jack, as he is in love with her himself. When the smelter's manager arrives and learns that a registered package of money will be arriving that evening, Barker snoops through the paperwork. Jack catches him, and the men engage in a fistfight. Later, Barker, the secret leader of the ore thieves, goes to the warehouse district and informs his men about that evening's delivery. That night, at Smelter Junction, Slippery and his men beat and tie up Jack, after which they lock him in a room with Rex. One of the bandits dresses up in Jack's uniform and forges Jack signature to sign for the registered package. Rex is able to loosen Jack's bonds, and the agent stumbles into the office just as executives from the smelter arrive to collect their package. Believing that Jack stole the money, the executives call Barker, and he orders Jack's arrest. Although Helen believes in her friend's innocence, he is convicted and sent to a prison work camp. Determined to solve the mystery, Helen assumes Jack's job. One day, Pepper informs Helen that the train currently at the junction has one car of explosives and one car of high-grade ore, which has been labeled Borax to fool the thieves. After Helen gives a waiting customer a telegram, Rex spills some ink, and on the telegram's duplicate, Helen sees that it is an encoded message telling the thieves about the hidden ore. Hoping to prevent the robbery, Helen switches the signs on the train cars, and the outlaws unwittingly release the brakes on the car of explosives so that it will roll down the track. Helen then learns that her father is coming to the junction, and that the rapidly rolling explosives car will collide with his train. Racing in an automobile, Helen reaches the car and manages to light one of the fuses, then jump to safety before the explosion, which signals her father's train to stop. Helen apprises her father of the events, and he tells her that the ringleader will be easy to track down, as few men knew about the hidden ore. Meanwhile, at the prison camp, a fire breaks out, and Jack escapes with Slippery, who has been jailed for failing to pay his alimony to his ex-wife. Slippery takes Jack to the gang's hideout, and there Helen, who has tracked the gang to the warehouse, overhears Jack trying to insinuate himself with them by claiming to want revenge against Pepper. Unknown to Helen, Jack is seeking evidence of Barker's guilt, and when she returns home, she brokenheartedly tells Pepper that Jack has joined the gang. When Jack appears in her room, Helen tells him that she is engaged to Barker and refuses to listen to his accusations. Meanwhile, downstairs, Pepper informs Barker that Helen has located the gang's headquarters, then overhears as Barker telephones Slippery, instructing him to pack their loot. Barker then attacks Pepper, but Rex alerts Helen, and she and Jack save Pepper. They then get the police and dash to the warehouse, where the men are loading a train. While the police are arresting some of the men, Slippery and his pal escape on the train. Helen and Jack jump aboard and gain control of the train, although when the fleeing Barker attempts to outrun the train in his car, he is run over in the crossroads and killed. Jack comforts Helen, and later, Pepper relaxes with Rex and watches approvingly as Jack and Helen embrace.