"Equestria's Guard: the Bravest and the Best. Do you have what it takes?"
So the advertisements say. When Rainbow Dash saves the life of Princess Luna, however, she quickly comes to learn that there's a lot more to the guard than what everypony knows. As she faces down threats to her friends, her Princesses, and her country, Rainbow will have to decide exactly who she is loyal to.
A Price of Loyalty story.