In Washington D.C., federal agent Rigby, Robert Taylor, is briefed on a racketeering ring that has been selling and retrofitting war surplus aircraft motors for profit off the coast of Central America. The government spots Tugwell 'Tug' Hintten, John Hodiak, and his wife Elizabeth Hintten, Ava Gardner, as part of the racket. Tasked with recovering the motors and raiding their headquarters, Rigby travels to Los Trancos on the island of Carlotta, posing as a sportsman. There, he arrives at a nightclub where Elizabeth sings. After she performs, Rigby enters her dressing room and shares drinks with Elizabeth Hintten and Tugwell, who passes out after a night of drinking.
Over the next few days, Rigby and Elizabeth begin a flirtatious romance. Rigby arrives at a bar where Charles Laughton J.J. Bealer offers him $10,000 to leave the island, but Rigby declines. Rigby also suspects Vincent Price Carwood is affiliated with Bealer. Regardless, he and Carwood venture on a fishing expedition the next day. Rigby catches a large fish, but the expedition turns deadly when their friend Emilio Tito Renaldo is killed by a shark. Emilio's father Pablo Gomez Martín Garralaga confirms that Rigby's suspicions that Carwood is involved and wants his son's killers brought to justice.
That same night, Elizabeth meets with Rigby and confesses her intention to divorce Tugwell. However, she learns that her husband has fallen ill. Rigby, in love with Elizabeth, becomes torn between his investigative duties and a desire to protect her from prosecution, all while declining Bealer's bribes. After an unsuccessful attempt to locate the headquarters, Bealer shows Rigby his X-ray photos confirming Elizabeth's involvement in the racket. Later on, Bealer tells Elizabeth that Rigby is a federal agent and schemes to drug him so they can leave the island with the money.
Elizabeth drugs Rigby's cocktail and he loses consciousness but she immediately feels remorse. Meanwhile, Carwood suffocates Tugwell for talking too much. Back in his hotel room, Rigby recovers and meets with Elizabeth at the nightclub. Backstage, he defuses a confrontation with Bealer, who states Carwood had murdered Tugwell. Rigby arranges with Bealer to have Carwood brought in to be arrested. When Bealer complies, a gunfight between Rigby and Carwood ensues. Carwood escapes and runs into a carnival parade, where he is fatally shot. Rigby returns to the hotel and kisses Elizabeth.
