Eight months after the East Street killing, the unsteady peace the citizens of Canterlot have enjoyed is shattered by the changeling invasion, heralding the return of two stallions who had been presumed dead ever since.
After a few months of consideration, Starlight comes to Twilight to request Cozy Glows release. Twilight agrees, although fear of how releasing one of Equestria's worst villainesses in the most turbulent of times causes her to hide the filly.
After an attempt on Twilight Sparkle's life at her coronation, the former Princesses decide it's time to call in on an old ally to protect the ruler of Equestria. In the meantime, an insurgency is rising up to challenge the new Princess.