The U.S. government asks Mr. Moto to go to Puerto Rico to investigate diamond smuggling after an earlier investigator is murdered.

Puerto Rico has become the back door by which a criminal syndicate smuggles inexpensive contraband diamonds from South America into the United States, dramatically driving down the price of legitimate gems. After they have ruthlessly murdered a special investigator, Moto is assigned to the case. While sailing to the island he impresses Twister McGurk, a slow-witted but amiable wrestler, with his martial arts abilities and gains a loyal friend and bodyguard. When an attempt on his life fails and another government official is murdered, Moto sends authorities a fake telegram identifying himself as a criminal named Shimura and making him and the Twister wanted fugitives. That ploy allows them to infiltrate the gang and expose the criminal mastermind behind it.—duke1029