Simon Lafontanne, head of the Lafontanne Chemical Company, goes to consult Charlie Chan who is staying in New Orleans along with his number two son, Tommy, and his chauffeur, Birmingham Brown. Lafontanne thinks that he has an enemy who is having him followed everywhere he goes. The trouble started when he entered into a partnership with two foreigners, Henri Castanaro and Theodore Von Scherbe, to ship a chemical to South America. After Chan agrees to investigate further at Lafontanne’s office the next morning, Lafontanne, who is about to enter his car, notices that the person driving is not his chauffeur. The phony chauffeur speeds off, as the real chauffeur appears, with the assistance of Tommy and Birmingham, showing the effects of having been hit over the head.
The next morning, at his office, Lafontanne’s secretary, Rene, who is also his niece, tells him that his partners in the chemical deal are waiting to speak with him. Castanaro and Von Scherbe are concerned about the possibility of one of them dying suddenly, so a clause has been added to their agreement whereby, in the event of the death of any of the principals, his share of the profits will go to the remaining partners. After Lafontanne reluctantly signs the clause, Oscar Swendstrom shows up at the office, brandishing a gun and claiming that Lafontanne had swindled him out of his formula for the chemical. Rene phones police captain Pete McNally to have Swendstrom removed, but when McNally and his assistant Dansiger arrive, they find Lafontanne dead in his office.
Chan then arrives for his appointment, and the police inform him of Lafontanne’s death, suspecting that he has died of a heart attack. Chan investigates and finds that the radio in Fontanne’s office, which had been playing music earlier, is now silent, except for a humming sound. The detective finds that one of the vacuum tubes is broken and takes it with him for further study, and the police take Swendstrom in for questioning.
Meanwhile, Tommy and Birmingham, after a lengthy search, locate Lafontanne’s car in a parking garage. Later, as Chan inspects the interior of the car, he finds some cigarette ashes containing coquina bark, a special ingredient peculiar to certain cigarettes, which is used for flavoring.
Later, after Chan has replicated the radio tube from Lafontanne’s office, he discovers that a certain sound frequency will cause the very thin glass to shatter. Chan then visits Castanaro who is holding a small party at his home. While he is there, Andre Pareaux and Nita Aguire, who are posing as a countess and her cousin, and who have an interest in diverting the chemical shipment for their own use, arrive. Suspecting their identities are not what they claim, Chan asks Pareaux for a cigarette in which he detects the presence of coquina bark.
Later, a letter arrives for Castanaro, which prompts him to phone McNally, reporting that he believes himself to be in great danger. However, by the time the police arrive at his home, Castanaro is dead under circumstances resembling those noted at Lafontanne’s office.
Chan, investigating records concerning Pareaux, finds that he has used various aliases besides Fernand (including Citroen, Boronoff, and Rombauer), and is apparently after the chemical formula.
Back at police headquarters, Swendstrom, under the pressure of interrogation, tells McNally that the murderer is Von Scherbe. However, when they arrive at Von Scherbe’s residence, he is found to be dead, murdered in the same manner as the previous two victims.
Chan returns to his house finding himself a prisoner of the gang that is out to steal the chemical shipment. Pareaux and Aguire, along with henchman Grock, had arrived earlier, knocking out Tommy and Birmingham and locking them in a closet. They now question Chan about his knowledge of the chemical shipment and Lafontanne’s formula. In response, Chan tells them about the recent murders. He tells them of how the victims died, and then he informs the group that the tube in his possession contains the same poison gas. He then plays a recording of a soprano singing a note which is of a pitch high enough to shatter the tube. Chan tells Pareaux and his associates that the gas is now in the room. Through his effective use of the power of suggestion, Chan convinces the members of the gang that they are beginning to feel the effects of the poison. As they panic, Tommy and Birmingham, who had heard Chan’s dialog while still in the closet, break into the room and surprise Chan’s captors. They also realize that the detective’s comments were a ruse, and that there is no poison gas.
McNally and his men arrive, arresting Parreaux and company, but Chan explains that it was indeed Swendstrom who had used the poison gas-filled vacuum tubes to kill his former business partner whom he felt had cheated him out of his formula. He was also responsible for the other two murders. Mrs. Swendstrom, who was in on the murders, had placed the tubes in the radios of the victims, and it was her singing voice on radio programs on which she had performed that had caused the tubes to shatter, thus releasing the deadly gas.