This episode is about Princess Luna and her ability to maintain and protect dreams of ponies everywhere in Equestria, but when her feeling turn dreams into a Nightmare that is just waiting to be free. She must forgive her past action with the help from Twilight and her friends, in order to stop it from turning Equestria into a living nightmare.
Callbacks from season 4 such as Flutterbat and powerponies.
Luna makes a comeback since her return in season 2.
Princess Alicorn Big Mac Is Now canon.
Ponies in Equestria have some weird dreams
A truly great Episode.