Pseudo-synopsis:       A mysterious criminal is stealing rare gems. The Earth Emerald housed in the Canterlot Natural History Museum disappeared recently. As has the Water Sapphire stored in the secure vaults of [no bank name yet] in Manehatten.  Now Rarity's Fire Ruby has gone missing, and Twilight must figure out what has happened to it. However, the Royal Guards, the police of the land, have turned a blind eye to the thefts. Twilight must delve into this mystery head first if she is to find, and stop, the grave danger Equestria is in. Along the way she meets one pony who shares her desire for justice, Sherclop Holmes.

General notes:
A mysterious criminal is stealing rare gems. The Earth Emerald housed in the Canterlot Natural History Museum disappeared recently. As has the Water Sapphire stored in the secure vaults of [no bank name yet] in Manehatten.  Now Rarity's Fire Ruby has gone missing, and Twilight must figure out what has happened to it. However, the Royal Guards, the police of the land, have turned a blind eye to the thefts. Twilight must delve into this mystery head first if she is to find, and stop, the grave danger Equestria is in. Along the way she meets one pony who shares her desire for justice, Sherclop Holmes.

So that's the gist of the backstory. Additionally, the Sky Diamond is been stolen from the Wonderbolts' headquarters in Cloudsdale. The Royal Guards are doing nothing about it, and Celestia has seemed to turn a blind eye to it. Twilight meets the one other pony who has made the connections and wants to see justice delivered, Sherclop Holmes.