When elderly Bertha Polacheck offers to burn down arson investigator Oscar Hollingsworth's bar so that he can collect on the insurance, Hollingsworth informs the Los Angeles Fire Department's Capt. Taplinger. Taplinger has just called in Robby "Rollie" Rollins to work undercover on the investigation, when another bar burns down. At the scene, Taplinger discovers the source of the blaze: a rag soaked in phosphorozene, a chemical made combustible by air. The investigators soon learn that the producer of the chemical, Hanley Chemical Company of Albany of New York, sells only to the United States government. After further investigation, R. J. McManus, the President of Hanley, fires one of his employees, Ralph Wessman, for stealing samples of the chemical to send to his brother, pharmacist E. V. Wessman. Taplinger and another investigator, Berkeley, decide to use Hollingworth's bar as bait to trap Wessman and Bertha and convince Wessman's assistant to quit so that rookie investigator Russ Haines can take the job. Soon Wessman receives a bottle of phosphorozene, sent by Ralph before he was fired. He hides it in the back room, and then tells Russ that he is stepping out for a moment. Once in the street, Wessman notices Russ sneaking into the back room and rushes back to discover the bottle empty. He then mixes a drink for Russ, which knocks him out, and by the time Russ wakes up, Bertha's sons Sam and Tony have arrived. Russ informs them that their connection at Hanley has been fired, and Russ explains that if they want to continue using phosphorozene, they must go through his brother-in-law, Dutch. After Tony and Sam agree to do business with Russ, he takes them to Hollingworth's bar where he and Rollie, who is posing as Dutch, agree to burn down Hollingsworth's bar that evening. When Tony and Sam insist that Russ light the bar on fire while they hide nearby, the arson team ignites a hidden gas line to mimic flames. Convinced that Russ is for real, Tony and Sam reveal Bertha's plans to burn down another bar soon. In the next day's newspapers, however, Tony and Sam read that the bar's sprinkler system extinguished the flames. Later, Tony and Sam request a pint of phosphorozene from the pharmacy, so Russ asks the police department chemist to manufacture an inert substance with the look and smell of phosphorozene. That evening, at the Polacheck home, Bertha asks her pet canaries whether or not they should go through with the plan. When the birds chirp a warning only Bertha can understand, Wessman suggests they target San Carlos Textile Company instead of the bar. After he rushes to inform detectives of the changed plan, Russ accompanies the rest of the gang to the textile company. When the imitation phosphorozene fails to ignite the building, Wessman accuses Russ of being a detective and the gang attacks him. They tie him up, ignite the building and leave him trapped inside. Russ soon regains consciousness and kicks out the window, setting off the alarm. After Russ is rescued by firefighters, Bertha, Wessman and the rest of the gang are sent to prison. For his brave work, Russ is given a permanent assignment at the arson bureau.