Twilight, after being found in a self induced isolation at a lunar prison, is taken in by Celestia as a student and to save Equestria from Nightmare Moon.
Then she finds out Luna isn't on the moon with her nightmare.
and then Luna finds out that Twilight, who is a part of a strange and powerful mercenary group know as the Night's Guard, just might be more of a nightmare than Nightmare Moon. In all the right ways, of course.
(new cover and description)