In Mexico City, when an attempt is made to steal the papers of the mysterious Alfred Wyans, his secretary, Dorn, who is working undercover with the United States government, begs Mexican police inspector Luis Caverro to send for Charlie Chan. The next day, Dorn is shot during a luncheon party at Wyans' house, but before he dies, he manages to type a few cryptic letters on his typewriter. After Chan, an old friend of Luis', arrives, accompanied by his third son Tommy and his assistant, Chattanooga Brown, Luis tells him that no gun was found at the scene of the murder and that, although the guests only heard one shot, two bullet holes were discovered. To Luis, Chan explains that Wyans' papers reveal the discovery of a new element that could be used in a bomb many times more powerful than the atomic bomb. Chan suspects one of the party guests, who include Marguerite Fontan, with whom Luis is in love; Edmund Slade, a former gunrunner; Joseph Bradish, an international smuggler; the Countess Irena, a singer; and Charles Masack, a Nazi propagandist. After he questions the guests, Chan accompanies Luis to the police lab and learns that the bullet that killed Dorn has no firing marks, indicating that it was not fired from a gun. Wyans complains to the police that his typewriter has been stolen, and Chan warns him to protect his papers. Later, Chan discloses that Marguerite has connections in Czechoslovakia, where the papers were stolen from the Allies by Wyans. While all the suspects are at dinner, Chan and Luis search Wyans' baggage, but are interrupted by the others, who all have the same idea. After the suspects leave, Chan and Luis discover the missing typewriter. Then Wyans tells them that he knows how Dorn was killed, but before he can reveal the answer, he is shot in the same manner as Dorn. At the nightclub where Irena sings, Chan reveals that she is not nobility, but is married to Charles Masack, who spies for several countries. Irena starts to reveal the secret, but she too is killed. After an attempt is made on Chan's life in a similar manner, Chan tries to discover why a bottle of indelible Chinese Red ink was found on Dorn's desk. Luis locates Chinese artist Iris Ling, who visited Wyans' hotel before Dorn's death. Iris tells Chan that she sold Wyans the ink to use on his "bandarillas." Chan surmises that the "bandarillas" were actually his typewriter ribbons. They examine the ribbon that Dorn removed from the typewriter before he was killed and discover on it the secret information written in indelible ink. Chan clears Marguerite from suspicion when he discloses that her uncle had discovered the secret element. He then explains that Bradish committed the murders to acquire the information, using a remote control device disguised as a thermostat that exploded the bullet that he had earlier slipped into his victims' pockets or purses.