Few ponies in Canterlot and Ponyville still suffer from the Blank Plague, but there's one very important exception. Princess Luna sequestered herself away from pony contact so she could raise the sun and the moon by herself while Celestia was ill. But Luna cannot live alone forever, so she must expose herself to the plague and endure its effects just as Celestia and Cadence did, while Celestia resumes commanding the sun and the moon once more.

Bellerophon Michaelides, the only human in Equestria, hence the only being immune to the magic-devouring plague, must tend to Luna as he tended to Celestia, with a recovering Twilight Sparkle staying by his side. But Luna has other duties beyond moving heavenly bodies, and dreams do not abide the light of day. Darkness does not rest lightly.