After reporter Pat Allen is killed at the Daily Sentinel by a leopard, Britt Reid finds a transmitter in a cigarette box. Later, D.A. Scanlon informs Britt of the discovery of a perfect diamond on Allen's desk.
Digging around, Reid discovers that the gemologist had been working on producing synthetic diamonds. As the Green Hornet, Reid goes to the house of the gemologist (Prof. Miller) who examined the diamond and demands to know how Miller makes the diamonds. Miller tries to kill the Green Hornet, but he and Kato follow miller to the house of Yolanda deLukens, who is in charge of her late husband's diamond company.
The Green Hornet demands 50% of the profit from the endeavor or he'll expose the entire operation. As a show of good faith the Hornet offers to get rid of Mike Axford who had been led to Yolanda's by notes in Pat Allen's apartment. Yolanda agrees, but places a transmitter in Mike's pocket and releases her pet leopard to finish him and the Green Hornet. Before the leopard can finish Axford, the Green Hornet sprays it and Axford with his Hornet Gas, knocking them both out. Later the cops bust the fake diamond ring and Mike is left confused about why the Green Hornet saved his life.