This story is a sequel to Matters of Principle
For as far back as she can remember, Vice Principal Luna has had no choice but to stand by and watch her well-meaning sister get in all sorts of trouble. Now, with supernatural threats appearing from another world, Luna finds herself losing faith in her ability to make any real difference in her students' lives, or Celestia's.
Perhaps a visit to that other world will provide some perspective.
Edits: nanashi_jones