Trixie is the most powerful unicorn in half a century, and has just become the cherished pupil of the illustrious Princess Celestia.
The Great and Powerful Trixie is the most renowned magician in all of equestria, or at the very least, the most boastful.
Sparkler is a pegasus stage-hand that's just trying to keep her younger sister from going hungry.
… Starlight is a retired thespian, a knackered-old mare obsessed with tragedy, trying to get her opus published.

Princess Luna is an alicorn with much to atone for in her life.

They are all connected by an ancient twist of fate.