Starlight Glimmer's plan to go back in time had backfired, and she turned herself into a foal and went back further than she intended. Now she has to find a way to return to her own time and into her adult body.
On the eve of Nightmare Moon's return three royal guards were transformed into toddlers. Celestia now has to raise these three foals and keep them in line.
After Mindset is told by Princess Luna to take a vacation from work to help the Block Kingdom, she learns that this place has a unique way of "relaxing" ponies. At least she'll fit in with the other foals...