Where did Celestia disappear to, the night of the 1,000th summer sun celebration? Why didn't Nightmare Moon simply kill the mane six? What was The Nightmare willing to do in its hatred, and The Princess willing to do for her beloved little ponies?...
Despite that everypony has forgiven her, Luna still lives with the guilt of her actions and it eventually builds up too much for her to live with. Worse, guilt is only part of it.
Celestia has a dream about Nightmare Moon, while Luna can do nothing but watch her sister's nightmare. When Celestia awakens, the two sisters talk about what they've just seen.
How could I have told you Celestia? How could I have explained to you that your beloved sister was now more damaged than she had been when she first returned? That her brief lapse into Nightmare Moon had left her so unimaginably broken?
With the help of her friends in Ponyville, the Tantabus was subdued...but what about Celestia? What about Luna's closest friend, a pony who had no idea what was going on in her little sister's mind?
Sisters are supposed share everything. Secrets, experiences, feelings, knowing silences... but sometimes first a silence must be broken for sisters to truly understand one another.
On the eve of the Summer Sun Festival Celestia has gone missing, and it's up to Luna to find her. What awaits Luna, however, is not only her sister, but a long awaited confrontation of her past.
Transition is never easy. And the past can be difficult to handle alone, especially as the mind wanders before sleep. But for the first time in a millennium, Luna is no longer alone.