While on a flight to Grenada, Connie Blair meets Mike Ingersoll, who confides that he is looking for sunken treasure from a ship that went down in the Caribbean.
 Mike shows Connie a tracing of the treasure map which shows the shipwreck location.

Coincidentally, Connie and Mike both are stopping off in Barbados where they have arranged to meet with a hotel manager whose hotel is a converted castle. When Connie and Mike meet Mr. John Marchant, the hotel owner, he is with Mr. Bertie Russell, an Englishman. She is suspicious of Bertie and tries unsuccessfully to warn Mike about discussing the sunken treasure in his presence. John takes Connie, Mike, and Bertie on a tour of the dungeon where Mike is almost killed by a falling heavy iron pot.

Connie arrives in Grenada to find that her luggage has been switched with Mike's. She tells Mr. Renshaw about the mix-up and that her suitcase is probably with Mike in St. Lucia. Connie returns to her hotel room after dinner to find an intruder, who escapes from the window. She realizes that the marauder was after Mike's map and goes after him, but doesn't overtake him. Connie returns to her room to discover that the map hasn't been stolen.

She flies by chartered plane with Bertie to St. Lucia and is knocked out. Connie regains consciousness and finds Mike, who says he doesn't have her suitcase. Later they discover that Mike's map has been stolen. Mike and Connie then go island hopping to recover their missing property. Mike is stabbed in a Martinique cathedral while Connie is off shopping. They return to Grenada without success to discover that Connie's suitcase has been found. They trap their assailant behind a waterfall and recover the stolen treasure map.