A young Pinkamena Diane Pie escapes from the rock farm where she and her family live and work after discovering her special talent. Now knowing that she is destined to be a pony of laughter and happiness, she arrives in Ponyville to find herself. She is, reluctantly, taken in by her grandmother as the Pie family on the rock farm is set into panic over the missing Pinkamena Diane. Bitter and angry over their own sister's escape, how she could just abandon them after teaching them how to laugh, Blinkie and Inkie plan their revenge.

UPDATE: Changed one of the categories from "Slice of Life" to "Adventure". Seems more fitting.

Wow, it's difficult to write a description without revealing the whole plot.
I'd love constructive criticism; what I did right, what I've done wrong, and anything else you'd like to share.