The Thea Sisters are just about to head off on their separate family vacations when they receive a call from Will Mystery. Once again, there is trouble in one of the imaginary kingdoms. A crack has been discovered in one of the maps in the Hall of Roses. The girls head to the Crystal Kingdom to help uncover the mystery. There, they find that a dragon has been wreaking havoc all over the kingdom. He has been turning all of their precious gems into stone and leaving a path of destruction where ever he goes. The sisters figure out why and help break the spell.

Geronimo Stilton is the pseudonym of the famous Italian writer Elisabetta Dami. She has written the famous series of books on Geronimo Stilton. She co-owns the Italian based publishing company called Edizioni Piemme. She likes to get all her ideas through her travels to places around the world.