Robby and the mysterious Mr. Lillibulero are back in a third adventure. This time the action centers around the wreck of La Floridana. The centuries-old Spanish sloop is being raised so it can be studied. Robby doesn't care about studying an old wooden ship but he hopes there might be treasure in the wreck. He dives into the wreck to have a quick look around for any gold or other loot, so he's on board La Floridana when it's stolen by the villainous Red Carswell! Robby is captured and held prisoner aboard Carswell's submarine.

Lillibulero is captured too. There's a truly delightful sequence where Robby escapes, then dons a disguise and boldly returns to rescue Lillibulero. They escape together, then Lillibulero chastises Robby for returning to rescue him. Turns out, Lillibulero had seen Robby being captured and had allowed himself to be captured as well, so that he could break out and rescue Robby. Robby complains at the double standard, but Lillibulero points out that he's trained for such situations whereas Robby is an amateur whose foolhardy rescue attempt put them both at additional risk.

In the end, Robby and Lillibulero steal back La Floridana and fight off Red Carswell's goons long enough to be rescued by the Coast Guard. Robby realizes that the real treasure isn't gold or silver, it is knowledge--the knowledge that will be gained by studying La Floridana.

I found this to be a stronger story than Secret Under Antarctica, but not so fun as Secret Under the Sea.