A Chinese detective solves a triple murder involving poison gas and its chemical manufacturers.
In San Francisco Lascari (Frank Bruno) tells Anton Mohl (Lucien Prival) that the ship carrying the chemicals is guarded. Anton calls Olga Petroff (Evelyn Brent). Simon Dayton (John Hamilton) asks James Lee Wong (Boris Karloff) to help him, because someone is trying to kill him over chemical shipments. Someone in Dayton's car nearly captures him after his chauffeur was attacked. Dayton's partners Theodore Meisel (William Gould) and Christian Wilk (Hooper Atchley) have him agree to inherit each other's shares. Carl Roemer (John St. Polis) insists on seeing Dayton to get his formula before the shipment. Dayton calls the police, and Captain Sam Street (Grant Withers) finds Dayton dead in his locked office. Dayton died of heart failure from poison gas, and Wong takes glass fragments to scientists. Wong asks secretary Myra Ross (Maxine Jennings) about Dayton. Street questions Miesel and Wilk. Anton tells Lascari and Olga they must get the formula. Street and Wong examine Dayton's car and go to Roemer's place to question Mrs. Roemer (Grace Wood). Wong experiments to learn what sound will make the glass break.
Wong calls on Wilk and meets Olga and Anton, whose cigarettes he notes match what he found in Dayton's car. Wilk gets a message from Roemer warning him of danger. Wilk calls Street, who finds him dead. Wong finds similar glass. Street and Wong question Roemer. Wong finds the criminal records of Olga, Anton, and Lascari while Roemer refuses to talk. Street has to release Miesel and the witnesses. Anton sees Wong in his apartment, and Myra makes Street release Mrs. Roemer. Miesel goes to dinner with Olga. Anton tells Olga he did not kill Wilk. Roemer tells police that Miesel killed both his partners. Police go to arrest Miesel and find him dead. Street concludes it was suicide. Wong asks Street to bring Roemer to his house. Wong finds Anton, Lascari, and Olga in his house. Wong says they tried to kidnap Dayton to stop the gas from going to their country's enemy. Wong shows the glass, and it is broken. Wong describes how the gas works and captures them as Street arrives with Roemer. Wong shows Street their FBI records and asks Roemer the method that Miesel used, getting him to show his guilt.
This mystery plays upon the subconscious fear of invisible poison gas that could be used in the next war as the astute and courteous Wong demonstrates to the police detective how to solve crimes.