Since the rise and fall of Nightmare Moon, Princess Celestia has wielded both the sun and moon herself. But she is incapable for such a task much longer, the strain is slowly killing her.

Princess Luna, now reformed and returned from chasing out the last of the Changelings, begs her sister to see sense, and give her back control of the moon, after saving her and Canterlot from a close accident.

But Celestia, on the verge of breakdown, refuses to oblige, still clinging to lingering doubts of her sister's Loyalty.
A bitter quarrel between them forces Luna to retire temporarily to Ponyville. Can she, Twilight and the other Mane Six, prove her loyalty, and bring Celestia to look reason in the eye? The next Summer Sun Celebration isn't far away, and Celestia's hold is weakening...