Inspector Warren of Scotland Yard flies into Jamaica and is taken to the headquarters of the Jamaica Constabulary Force. Exchanging credentials with a Jamaican Inspector, Warren reveals he has come to seek the extradition of a wanted criminal known as "The Professor". Surprised when the Inspector refuses to extradite the Professor, the Jamaican Inspector recounts a story told in flashback.
Devil-may-care adventurer Brannigan has won the ship Manfish and its first mate Swede in a poker game but keeps away from the Manfish's creditors. During a drunken evening out Brannigan is attracted to a woman sitting with an older man called "the Professor" with the two brawling over her. During the brief fracas Brannigan notices the Professor is wearing an unusual heavy ring of a skull and cross bones.
The next day the Manfish is at sea engaging in a turtle hunt. Two divers from the ship find a skeleton underwater holding a bottle. Brannigan swims to the skeleton and grabs the bottle. He breaks it open on board finding a ripped piece of paper dated 1793 with nonsensical French written on it, but inside the bottle is the same ring that the Professor wears.
Tracking down the Professor, Brannigan persuades him to tell what he knows revealing it is half of a document by the pirate Jean Lafitte with the professor holding the other half.
Fearing for his life but clever and greedy, the Professor translates the map to reveal a treasure is on the island of Hispaniola but the Professor insures his safety by destroying the map and memorising the contents. Once on the island a treasure worth £25,000 is recovered but the Professor promises it is a drop in the ocean compared to another treasure buried later by Lafitte that the Professor says he can locate.
