At her elder sister's advice, Princess Luna decides to take on a student of her own, offering a misunderstood young unicorn mare named Obsidian the chance of a lifetime. Obsidian gratefully accepts the honor, but gets a little more than she bargained for.

Even though Nightmare Moon was defeated, the Nightmare is far from over. The very same dark force that once corrupted Luna is now taking a special interest in Luna's new protégé.