Twelve year olds Aggie Morton and friend Hector Perot head out for Christmas to her older sister Marjorie's house (at Owl Park) for this cozy mystery. 

Fellow guests at the manor, Mr. & Mrs. Sivam, reveal an enormous emerald, the Echo Emerald,  that's said to be cursed because it was stolen from a Ceylon (once a British colony, today part of Sri Lanka) goddess' temple. Mr. Sivam is keeping it hidden until he can return it and lift the curse, but he's arranged for an appraisal before the journey to take place at Owl Park. 

Marjorie's hired a troupe of traveling actors (it's set in the days before TV or Internet) to "perform" a series of still-life re-enacted scenes from their favorite books, called a tableaux. The scenes are from, appropriately, a Sherlock Holmes tale, A Christmas Carol, and Long John Silver.

On Christmas day, the kids begin the house tradition of following clues to find their stuffed stockings, when they come upon a dead body in the library, face down in a puddle of blood and stabbed, or so it seems, with the actors' sword. And then Mr. Sivam and the emerald go missing.

Are the two events linked? It's up to Aggie and Hector to solve the mystery, along with Aggie's almost-cousin Lucy.

This is a fun, Christmas who-dun-it middle grade read!  Enjoy.