After reading this book, I was a little surprised to find that the Treasure of Lima is an actual, and it really is supposed to be buried on Cocos Island. I had assumed, while reading the book, that it was made up for the book.
Annja is on vacation (boy, she goes on a lot of those through the series) in Costa Rica. There she meets a woman whose archaeologist husband has disappeared on Cocos island while looking for the treasure of Lima, and the government isn't allowing her to go looking for him. Through Roux, Annja pulls some string, and goes with the woman and her hired hands to find the man. Unfortunately, despite her research before heading out, she doesn't bother doing any research on the people she is travelling with (a failing she does kick herself for later)
It's also a little hard to believe, in this day and age, that there is a hidden Incan outpost still surviving on this island (albeit underground), but still, it was a fun 'lost world' type of story.
The weakest part of the book for me was the villain, who was a little too cartoony for my tastes when revealed.