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Bertram Lynch, a peanut-munching, seemingly slow-witted investigator, arrives in the British Crown Colony of Trinidad with his pet monkey after Sir Ellery Bronson, governor of the island, sends for him to take over the investigation of stolen Brazilian diamonds that are being smuggled through Trinidad. Howard Sutter, the head of a private interest leasing government oil lands, and Inspector R. Gordon Henley have been assisting Major Bruce Cassell, whom the governor removed from the case. As Cassell's daughter Joan prepares for a dinner in honor of her sweetheart Gregory, Sir Ellery's nephew, she finds a few diamonds hidden in her father's bureau. She tries to convince her father to resign on account of his health and go with her to Europe, but Cassell is adamant on continuing until the investigation is over. Outside the dinner, Gregory warns Lynch just in time for him to avoid a dagger thrown by a cloaked figure. Lynch then finds his young assistant Graham, who had been working undercover for months, dead and discovers a note alerting him to investigate Queechie, a large black man living in a shack on the edge of the Caroni swamp. The dagger, similar to one which Sir Ellery's secretary, Gookol Meah, once gave him as a gift, has on its handle a snake and crescent insignia, the symbol of Blackbeard Teach, a pirate from an earlier era. Later, when Sir Ellery is found stabbed through the heart, the dagger given to him by Meah is discovered to be missing, but Meah attests that the murder weapon, which has Cassell's fingerprints on it, was wider and heavier. Henley then explains that Cassell, having heard cries of help, found Sir Ellery and tried to take the knife out of the body. When they go to interrogate Cassell, they find that he and Joan have left. Lynch and Gregory, who is greatly concerned about Joan, find Queechie and after convincing him that they are wanted men hiding from the police, pay him to take them to a hiding place. Queechie then loses them in the swamp. The next day, two men take them to a camp, which is run by Duval, who carries a dagger with the snake and crescent moon insignia, and they find Joan there. Lynch deduces that Duval is not the murderer, but that he is a smuggler working with someone higher up. At night, after Lynch sees Duval and his man Getchel leave in a motorboat, Joan tells him and Gregory that her sick father did not kill Sir Ellery, and that after they left because they feared that he would be arrested, they were forced to go with Duval. Joan relates that the diamonds found in her father's room were placed there by someone else and Lynch says he believes her. When Duval returns, Gregory and Lynch escape in a rowboat after a fight. They return in a police boat and see Duval in his boat attempting to leave the swamp. Although Lynch announces that he wants Duval taken alive, Sutter shoots Duval. After Gregory finds Joan and her father below on Duval's boat, Lynch reports that Duval is still alive. That night, someone sneaks up to Duval's hospital room and throws a dagger into his chest. The man is caught and found to be Sutter, whereupon Lynch reveals that Duval actually had been killed instantly by Sutter's bullet, but Lynch had perpetrated the ruse to draw out the real murderer.