Two murders occur on a train shipment of silk because of efforts to keep a corner on the New York silk market.
Use of silk is increasing, and Wallace Myton (Arthur Hoyl) corners the market to drive prices up. Kilgore (Neil Hamilton) calls in Myton and says he will import silk from Japan to Seattle. Kilgore goes there and charters a train that is loaded. Conductor Clark (Arthur Byron) instructs guards Craft (Harold Huber) and Burns (G. Pat Collins). They were hired by Myton, who says he also has an agent on the train. Attorney Calhoun (Robert Barrat) reports to Kilgore. A USDA inspector finds Mongolian silk rust, but Calhoun says he can only hold those bails. Craft knocks out a man, and Burns puts him in a car, saying he is dead. Dr. Ralph (Vernon Steele) and Paula Nyberg (Sheila Terry) ask Kilgore to take her sick father (Dudley Digges) to a specialist. Archaeologist Nyberg says he suffers from an ancient fly bite that is paralyzing him. Clark says the train may make New York in 73 hours.
Dr. Ralph sees a car on fire, and Clark stops the train. They put out the fire and find the corpse, Kilgore's secretary. Craft and Burns realize that someone else killed him. A note is tossed out to report the murder, and detective McDuff (Guy Kibbee) stops the train and asks questions. He says the death was only an hour ago. McDuff has everyone empty their pockets. Professor Nyberg has a Thug charm of assassins, and McDuff says that Dr. Ralph's letter from the governor was forged. Craft and Burns find the tramp Rusty (Allen Jenkins) and bring him to McDuff. Kilgore tells Craft and Burns to lock up McDuff so the train can move. They do it, hoping to expose Myton's agent. Dr. Ralph says that Nyberg has lost all his senses but sight. A note is tossed out. Three hours from New York Kilgore releases McDuff, who says that Kilgore or Clark wrote the note. Clark is found dead, but Dr. Ralph can't say how. McDuff asks where each person was.
Myton says the train will be impounded in New York. Professor Nyberg is questioned by Paula writing, signaling with his eyes. In a tunnel it goes dark, and Kilgore catches Craft with an icicle, which he says is how Clark was killed. Burns pulls a gun; but Rusty is behind him and Craft with a gun. Police let the Nybergs go. Rusty says that he is from Lloyd's Insurance to protect the silk shipment, and Calhoun gets the silk released.
This murder mystery reflects the dangers of business greed in the era of rapid rail transport. A sharp lawyer proves necessary for defense.