The police are unable to abate a crime wave that has hit their big city. Nightclub owner Jake Pelotti, meanwhile, wins $20,000 at the racetrack, but his wife is forced to use the winnings to pay ransom to Pelotti's competitor, Biff, when he kidnaps him. After he is released, Pelotti is angry that Biff has taken his money, and kills Biff's thug, Tony. Professor Cosmo Jones, a correspondence course detective, is walking in front of the police station when Tony's body is thrown from a car. Cosmo reports the finding to police chief Murphy, who is under pressure from the police commissioner to clean up the city. Cosmo wants to be hired as a special investigator, but Murphy refuses. After Cosmo accepts a ride from Sergeant Pat Flanagan, Pat averts the kidnapping of Phyllis Blake, the daughter of wealthy oil tycoon, James J. Blake. Cosmo stays behind to investigate and encounters a janitor named Eustace Smith, who has found the body of Mike Andrews, a bystander who was injured during the attempted kidnapping. To avoid her father's wrath, Phyllis and her boyfriend, Tommy Hayes, deny that she was almost kidnapped, and Pat is demoted for shooting Mike. As Pat refuses to marry his fiancée Susan, the commissioner's secretary, until he is promoted, she helps Cosmo with his investigation, and Cosmo makes Eustace his assistant. Tommy, who secretly works for Biff, later succeeds in kidnapping Phyllis, and her outraged father demands a shakeup of the police department after three days of apparent inaction. Pat and Cosmo join forces and obtain Blake's cooperation to bring the kidnappers out into the open by publishing a false report of a ransom note. Biff murders Pelotti, believing that he was trying to cut in on his action, and then sends an actual ransom demand. Hoping to protect Blake, Cosmo gives him a satchel containing counterfeit money, but Blake abandons Cosmo when Biff makes a new arrangement. Cosmo then accidentally switches satchels with a doctor at a drugstore, and leaves a note there for Murphy and Pat with the address of the new meeting place. Cosmo and Eustace are abducted by the gangsters and interrogated along with Blake. Biff, angry that there is no money in either satchel, threatens to deface Eustace with a scalpel, until Pat and Murphy arrive and fire at him. While Biff and his thugs return the fire, Cosmo and Blake rescue Phyllis. Eustace knocks one thug out while Pat shoots Biff in self-defense, and the gangsters are arrested. The commissioner thanks Cosmo for his diligence, and with the promise of a promotion, Pat can now marry Susan.