This story is a sequel to On Second Thought...
After becoming the new rulers of Equestria, Princess Twilight Sparkle and her friends found themselves trapped in Tartarus easily, thanks to a plan from Empress Cozy Glow.
Why? Because the Mane 6 and Royal Adviser Spike gained the "uselessness" of ex-Princess Celestia and ex-Princess Luna.
Now, the only ponies that can save Equestria from Lord Tirek, Queen Chrysalis, Grogar, and Empress Cozy Glow, or the "Legion of Cozy" are Starlight Glimmer and Trixie Lulamoon. Really, it's all up to Starlight.
Will Starlight be able to find the power inside herself to defeat the Legion of Cozy? Or will the LoC be victorious? Starlight may be very surprised about what will eventually happen to her...
Enough will be explained to not require reading the prequel to understand this story (and laugh) but it would answer why Twilight and the others took over for Celestia and Luna, and it took place right after the season 9 premiere.
At the time this story was written, I didn't know what will happen in the season 9 finale of MLP:FIM, so there will be no actual spoilers for it. Any similarities between this AU story and the series finale is 100% coincidental.