When Twilight's portals release Tirek into the human world and the humans get turned into ponies, they have to unite with their ponyselves to defeat him and save both world's.
Sunny Flare gets her first report card of Principal Cadance's tenure, and it's not good. Her mother, former-Principal Abacus Cinch, has an uncomfortable explanation for that.
In an alternate timeline, somepony else got to the Mirror Portal before Sunset and stole both it and Celestia away. Acting Princess Cadance is willing to give Sunset a second chance, and hopes to have Sunset's help with a matter of national security.