When an official from Britain’s Scotland Yard arrives in Jamaica to extradite a criminal, known there as “Professor,” to stand trial in Britain on embezzlement charges, the local police chief tells him why he is unable to hand over the professor: Brannigan, the rough-edged captain of the fishing boat Manfish , is in debt to Bianco, a local Jamaican businessman. After Brannigan attempts to pick up Alita, the young girl friend of a man known only as "Professor," he and the professor become involved in a brawl. The next day, Brannigan sets out on the Manfish with his first mate “Swede” and two divers, Domingo and “Big Boy,” to fish for turtles. The scuba divers are successful, but after an encounter with a shark, they are further shaken when they come upon a human skeleton on the ocean floor. When Brannigan dives to investigate, he discovers that the skeleton is holding a bottle in its hand. Upon returning to the surface, Brannigan breaks open the bottle and finds a map, in French, and a ring inside. He recognizes the ring as matching one that the professor wears. Back in port, Brannigan asks his girl friend Mimi, who understands French, to look at the map and she tells him that it appears to be only part of a map containing highly coded clues to the location of a buried treasure. After Brannigan and Swede locate the professor on a nearby island, the professor tells him that he found his ring in a nearby cove. When Brannigan dives there, the professor follows him and attempts to kill him with a spear gun. The ensuing fight between the two men is cut short with the arrival of Alita, who admits that they have the other part of the map. The professor then agrees that they should become partners with Brannigan. On board the Manfish , the professor tells Brannigan that the map belonged to the notorious, French pirate, Jean Lafitte, and that it uses riddles, mathematical formulae and the two rings to conceal the location of Lafitte’s buried treasure. Being better educated than Brannigan, the professor states that he will attempt to break the codes. The next day, however, Brannigan is dismayed to discover that the professor, in order to ensure that Brannigan does not kill him, has memorized the entire map and then burned it. Brannigan, the professor, Alita, who is playing both men against each other, and Swede then sail to an island where the professor suspects the treasure is hidden. After deciphering the map’s various codes and clues, the professor tells Brannigan and Swede to dig at a specific spot and soon they unearth a small chest full of gold jewelry. The professor then advises Brannigan that there is another map under the gold with clues to a bigger treasure. Before seeking that chest, however, they decide to return to port where Swede is assigned to sell some of the trinkets to raise more money. When Swede and Alita leave the boat to meet Aleppo, their potential buyer, the professor kills Brannigan with his spear gun and temporarily conceals the body until he can dump it overboard. Aleppo tells Swede that Brannigan is likely to lose the boat, Swede's only love, as Bianco is filing papers to seize it. After several interruptions, including the arrival of Mimi looking for Brannigan, the professor eventually slips the body overboard. However, when an oxygen cylinder, used to weigh the body down, fractures slightly on impact with the ocean floor, a small stream of bubbles begins to rise to the surface, alarming the professor. The professor then ties a rope around the treasure chest, hides it and, upon Swede’s return, tells him that Brannigan has left with the chest and its contents and has deeded the boat to Swede. Swede does not believe him, however, and refuses to move the boat from the harbor. Meanwhile, the incessant sound of the bubbles rising to the surface maddens the professor. The next day, when Bianco arrives with a magistrate’s order allowing him to seize the boat, Swede notices that an air tank is missing, spots the bubbles, dives in and recovers Brannigan’s body. After the now insane professor is arrested and dragged away, Alita gives one of the gold bracelets to Mimi, saying that Brannigan had wanted her to have it, but Mimi throws it away. The money Swede received from Aleppo covers Brannigan’s debt to Bianco, who then gives the boat to Swede. As Swede and his diver friends leave harbor, Swede cuts a rope that is fouling the propeller, and the chest, which the professor had hidden under the boat, sinks to the bottom.