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Simon Lafontanne, head of the Lafontanne Chemical Co., goes to consult with Chinese-American private detective Charlie Chan, as he 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 Castanero and Theodore Von Scherbe, to ship a chemical to South America. After Chan agrees to investigate further at Lafontanne's office, Lafontanne discovers that his chauffeur has been beaten up and his car stolen. The next morning, at his office, Lafontanne's secretary René, who is also his niece, tells him that his new partners are waiting to speak with him. Castanero and Von Scherbe are concerned about the possibility of one of them dying suddenly and have added a clause to their agreement, whereby in the event of the death of any of the principals, his share will revert to the survivors. After Lafontanne signs the clause reluctantly, Oscar Swendstrom shows up at the office, brandishing a gun and claiming that Lafontanne stole his formula for the chemical. René phones police captain Pete McNally to have Swendstrom arrested, but when McNally and his assistant, Dansiger, arrive, they find Lafontanne dead in his office. Chan then shows up to keep his appointment, and the police tell him that they think Lafontanne may have died from a heart attack. Although Chan investigates and finds a shattered tube in the office radio, the police take Swendstrom in for questioning. Meanwhile, Tommy, Chan's "number two" son, and chauffeur Birmingham Brown locate Lafontanne's car. Chan looks it over and finds some cigarette ash containing traces of a Mexican bark. Later, Chan discovers that a certain sound frequency will cause the glass in radio tubes to shatter. Chan then visits Castanero, and while he is there, André Pareaux and Nita Aguirre, who have an interest in diverting the chemical shipment for their own use, arrive. Pareaux offers Chan one of his specially made cigarettes, in which Chan detects the same material he found in Lafontanne's car. Later, a letter is delivered to Castanero, which prompts him to phone McNally to report that he is in great danger. However, by the time the police arrive, Castanero is dead under circumstances similar to Lafontanne's demise. Chan investigates Pareaux and finds that he has had various aliases and is apparently after the formula. Swendstrom finally tells McNally that Von Scherbe is the murderer, then he, too, is murdered. All of the suspects in the case are invited to Chan's house. Pareaux and Aguirre, along with henchman Grock, arrive early and knock out Birmingham and Tommy, then demand information from Chan about the Lafontanne deal and the formula. In response, Chan tells them that a radio tube he is holding contains a deadly poison gas. He then plays a recording of a soprano singing a note which is of a pitch high enough to shatter the glass. Chan tells Pareaux and his associates that the gas is in the room and they panic. Tommy and Birmingham break in, but discover that there was no poison gas in the tube. McNally and his men arrive and arrest Pareaux and company, but Chan explains that it was Swendstrom who had used the gas in the tubes to kill his former business partner and who had also carried out the subsequent murders. Swendstrom's wife was in on the crimes as it was her singing voice on the radio programs that triggered the shattering of the tubes.