When Michael Lanyard, the reformed jewel thief known as "The Lone Wolf," and his butler Jamison accidentally set off a bank alarm while chasing a stray cat, Police inspector Crane, Lanyard's nemesis, bets him two months' salary that he cannot stay out of trouble for twenty-four hours. After accepting the bet, Jamison and Lanyard return to their hotel. Outside the building, Johnny Baker, the inventor of a burglar-proof mail car, meets a private detective and shows him a telegram from Johnny's fiancée, film actress Gloria Foster, asking him to meet her in room 909 of the hotel. Suspicious about the telegram, Johnny and the detective proceed to the room and there are assaulted by two men. Climbing onto the fire escape, the private detective knocks on the window of the adjoining room for help. Lanyard, the room's occupant, thinks that it is an elaborate ruse concocted by Crane to win the bet and ignores the plea until he hears a gunshot and the detective falls to the ground, dead. Sneaking into the neighboring room, Lanyard witnesses Johnny being led away at gunpoint and finds Gloria's telegram on the floor. Realizing that Johnny is the only man who can exonerate him from the charge of murdering the detective, Lanyard sends Jamison to follow them. After Crane issues a warrant for his arrest, Lanyard escapes down the hotel's laundry chute and goes to the theater where Gloria is to perform. While waiting to see Gloria, Lanyard watches a newsreel featuring Johnny's burglar-proof car. When the film's narrator reports that the car will be carrying treasury engraving plates on its maiden voyage to San Francisco, and continues that if the proper combination, known only to Johnny, is not employed to open the car, poison gas will flood it, Lanyard realizes why Johnny was abducted. Lanyard then explains Johnny's peril to Gloria, and after Jamison informs him that he has tracked the men to the train station, Gloria offers to drive Lanyard to Perrytown, the next stop. After Johnny's abductors, Frank Jordan and Vic Hilton, tie Johnny to a stretcher and board the train, Jordan claims that Johnny is his sick brother and Hilton is their doctor. At Perrytown, Gloria and Lanyard meet Jamison on the train. The conductor, however, recognizes Lanyard from his picture in the paper and notifies Crane. As Lanyard attempts to sneak into the compartment to free Johnny, Jordan accosts him and alerts the conductor to Lanyard's intrusion. When Lanyard challenges Jordan's claim that the man on the stretcher is his sick brother, the conductor summons Dr. Hooper Tupman, a passenger in the adjoining compartment, to verify the diagnosis. After Tupman, who is in league with the others, confirms the patient's illness, the conductor takes Lanyard and Jamison into custody, intending to turn them over to Crane at Coasterville, the next stop. Tupman overhears their conversation and decides to remove the plates and detrain before the police arrive. After the thieves kidnap Gloria from her compartment to prevent her from corroborating Lanyard's story, they leave the train with the stretcher and board a waiting ambulance. Soon after, Crane arrives and when Lanyard insists that Gloria can exonerate them, Crane knocks at the door of her compartment. Evelyn Jordan, one of the gang, opens the door and posing as Gloria, denies any acquaintance with Lanyard. Escaping once again, Lanyard and Jamison commandeer Crane's taxi after Evelyn and the others speed away in the ambulance. After their car runs off the road on a hairpin turn, Lanyard and Jamison continue on foot in the midst of a howling windstorm. Tracking the thieves to an abandoned house, they enter the building and discover a secret passage after Jamison falls through some loose floorboards near the front stairs. Following the passage, Jamison and Lanyard find the room in which Gloria is being guarded by Hilton. After overpowering Hilton, Lanyard sends Jamison and Gloria to town to find Crane. They arrive too late, however, because Crane has already traced Lanyard to the house. When Lanyard insists that Hilton can prove his story, they proceed to the room where Lanyard left Hilton, but he is now missing. Crane then arrests Lanyard, but as he accompanies him to the front door, the two men fall through the loose floor boards. After boosting Crane back upstairs, Lanyard escapes through the passageway and discovers the thieves' cellar hideout, where he is taken prisoner. When Lanyard watches Tupman extract the plates from the stretcher, he realizes that the thieves have locked Johnny inside his burglar-proof car, which is still onboard the train. Jamison, who has returned from town, meanwhile, removes a plug from the ambulance's engine, and when the vehicle won't start, the thieves stash the plates in the well. In the chaos, Lanyard escapes and after Jamison reattaches the plug and retrieves the plates, they speed away in the ambulance, leaving the thieves stranded along the roadside. When Jamison tells Lanyard that the police intend to break open the car at Gary, the next stop, Lanyard realizes that they must free Johnny, their only witness, before he is asphyxiated by poison gas. Noticing that the newsreel is playing at a local theater, Lanyard goes there and forces the projectionist to magnify the image so that he can read the combination as Johnny turn the dials. Meanwhile, the thieves, who have been picked up by Crane, protest their innocence. Chartering a plane to fly to Morgan's Junction, Lanyard boards the train there and frantically dials the combination to the car. Soon after, the train pulls into Gary and the workers begin to break open the car. As poison gas pours out, Lanyard uses the right combination, freeing Johnny in the nick of time. After Gloria and Johnny are reuinted, Jamison returns the plates to Crane and pays his debt with a stack of freshly printed banknotes.