A police scientist investigates the mayor's murder.

With the help of Gerald I. Ladimer, a slick young attorney, Mayor Daniels and Turnley, the district attorney are elected on a reform ticket to smash the rackets that have been dominating their city. Unknown to the new mayor and district attorney, Jerry is in league with underworld boss Matty, who has promised to back him for the United States Senate in return for political favors. When Jerry realizes that the district attorney is serious about instituting reform, he notifies Matty and soon after, the district attorney is found murdered. After the mayor appoints him special prosecutor, Jerry visits the crime laboratory run by his old friend, chemist Gordon McKay and his assistant, Jane Mitchell. By meticulously piecing together the clues, Gordon determines that Turnley's killer is Allison Stacy. After Stacy is killed while resisting arrest, Eddie Wright, the owner of a small restaurant that has been victimized by the racketeers, determines to bring the criminals to justice and goes to the mayor's house to volunteer his help. There, Eddie is mistaken for a prowler and interrogated by the police. The mayor, meanwhile, summons Jerry to his office to question him about an expensive insurance policy he has just purchased. When the mayor demands to know the source of the money used to buy the policy, Jerry, fearful that an investigation would uncover his underworld connections, builds a bomb and plants it in the mayor's car. After the mayor is killed in the explosion, Eddie is arrested as a suspect. When threads from Eddie's coat match those found in the bombing, Jerry urges his arrest for murder, but Gordon remains skeptical. As the investigation progresses, Jerry falls in love with Jane and proposes to her, but she replies that she cannot quit her job until the murder is solved. Upon learning from Jane that Gordon has determined the bomber would have gun powder embedded under his nails, Jerry assiduously scrubs his hands to remove all traces of gun powder. One day, while examining some documents that he removed from the mayor's office, Gordon finds a remnant of a penciled notation regarding Jerry and deduces that Jerry must have murdered the mayor. To prove his suspicions, Gordon summons Jerry to his laboratory, then tricks him into a hair test that establishes his guilt. Later, Gordon informs Jane that he has solved the case by means of a hair sample, but refuses to divulge the killer's identity. Jane then meets Jerry at his house and after accepting his proposal, tells him that the case has been solved and that Gordon has traced the murderer through a hair sample. Realizing that Gordon has determined that he is the murderer, Jerry sets up an emergency meeting with Matty and hands Jane his car keys to drive herself home. Jane, who has been tracking down nail cutters as one of the clues, notices a cigar cutter attached to Jerry's key chain. Thinking that the cigar cutter may have been used to clip the bomb wires, Jane speeds to the lab to examine it. Jerry, meanwhile, meets with Matty, who hands him a pistol and instructs him to kill Gordon. As Jane inspects the cigar cutter in an interior room of the lab, Jerry bursts into Gordon's office, gun in hand, and demands the evidence. Hearing the commotion, Jane walks in just in time to hear Jerry admit that he killed the mayor. Gordon then jumps Jerry and overpowers him just as the police arrive. After Matty and his gang are arrested, Gordon realizes that he is in love with Jane, and when he proposes to her, she gladly accepts.