This is a 20th century version of Treasure Island with aeroplanes, hurricaines, galleons, treasure (natch), gun battles and good stiff-upper-lip derring-do. In my limited view, the best Biggles story ever. When I was 10 years old our teacher read it to us at school; then I bought it and read it at least twice more. I have read it to my 3 sons in turn. Biggles flies North and Biggles Flies South are up there as well (he flew all over the place) but this is the most relentlessly exciting story.
The prologue gives some historical background of a pirate ship on the Spanish Main, captained by the notorious Louis "The Exterminator" Le Grand who captures a ship only to discover that in the horde is a cursed doubloon that spells bad luck to those who have it in their possession. Louis contracts the plague (Y. pestis), his crew mutiny and abandon him aboard the ship which runs aground on a Carribean island. Louis recovers and, determined to rid himself of "Bawn's accursed doubloon", drags all the gold off the ship and hides it, save one coin that somehow gets trapped in his clothing and he allows this to get the better of him.
We then come forward to the mid 20th century and a sailor falls into the rotting ship whilst being chased. Falling ill after being captured he sends a letter to his son, Dick, a 15 year old Londoner, explaining about the ship and enclosing a doubloon that he found there. Dick nearly gets run over by a lorry and is saved by a passing Biggles who takes Dick under his wing. The ensuing adventure takes Biggles, Algy, Ginger and Dick to the Caribbean in search of the treasure while dogged by a) the villain who'd chased Dick's father and b) inexplicable bad luck all the way.