A speeding train becomes the setting for murder.

In Los Angeles, telephone operator Ruth Raymond learns from lawyer Alden Murray that she actually is the long-lost daughter of railroad magnate Luke Carson. Carson had been kidnapped as a toddler by his vengeful brother and business partner Elwood and reared in obscurity by strangers. Soon after Ruth is established as an heiress, her bodyguard and chauffeur attempt to kidnap her, but are foiled by the arrival of the mysterious Godfrey D. Scott, who deliberately punctures the kidnappers' gas tank while Ruth is meeting with her boyfriend, John Blake, in a park. After the failed kidnapping, a telegram from Carson, in which he tells Ruth that he is meeting her in Los Angeles, is intercepted and replaced by a telegram that advises Ruth to take the train to New York. As Ruth, her best friend Georgia Latham, and Murray board the private car of one of her father's east-bound trains, John rushes to warn Ruth about the trip, but is chased away by the police. Soon after the train leaves, the lights go out in Ruth's car, and she finds a note that reads "eight hours to live." At that moment, Scott announces his presence on the train and introduces himself to the group as a "de-flector," a sleuth who prevents crime. While Scott defends himself to the doubting Ruth, Georgia and Murray, a train porter is thrown from the train by an unseen assailant. Allen, Ruth's adoring former boss, then storms into the car and, aware that Carson is not in New York, accuses Murray of duplicity and insists that Ruth is not Carson's daughter. After quieting Allen, Scott reunites stowaway John with Ruth, and romances Georgia until the train is stopped suddenly by a circus wreck on the tracks. While the wreck is cleared, a gorilla escapes from its cage and sneaks into Ruth's car. Later, Murray is stabbed to death, and a voice announces that the remaining private car passengers only have five hours to live. The gorilla then attacks Ruth and Georgia and finally jumps from the train after attacking a terrified Scott. The next morning, the train stops in a small southwestern town, where Carson had been told through a strange radio message to meet up with Ruth. After father and daughter enjoy a happy reunion, they re-board the private car and resume their journey. Suddenly all of the windows are blackened one by one, and the mysterious voice tells the frightened passengers to "say their prayers." Carson finally recognizes the voice as that of his brother Elwood, and once identified, Elwood explains that as revenge for Carson's previous defrauding, he has planted explosives on the private car and uncoupled it so that it is now on a collision course with an east-bound train. As the train hurtles backward through the mountain divide, Scott overwhelms and kills Elwood, who is revealed to be in the disguise of Hanks, a porter, then radios the other fast-approaching train. After a long and treacherous trip, the private car avoids the collision, and all of its passengers are transferred safely to another train.