Sunset Shimmer's diplomatic visit doesn't go quite as planned. Especially when nudged off the rails by a lot of booze, some minor flirting, and an entire truckload of bad decisions.
Sunset Shimmer tries to explain to Princess Celestia what happened at her CSGU class reunion. And this time everything is very, definitely, not her fault. At all.
It turns out that Sunset Shimmer's final letters to Princess Celestia before her exile were not filled with resentment or remorse. There might have been a lot more frantic running away involved. But it was definitely not Sunset's fault. At all.
Sunset Shimmer visits the Castle of Friendship, and inadvertently discovers something that Twilight would normally keep hidden under her bed... except that since this is a pony princess we're talking about, it's nothing remotely resembling normal.
Equestria is having a little problem - there are starting to be way, way too many alicorns! Sugarcoat investigates and makes a lot of very awkward discoveries - namely that it's all Cadance's fault. .
One day, Twilight Sparkle discovers something amazing about her friend Sunset Shimmer. She's a magical talking pony! This requires investigation! Science! Possibly a full-body physical! Or maybe she'll just ask her what's up with that.
When ponies talk about Trixie, all they can see is the showmare. So many of them just dismiss her as a braggart and a fraud. They don't know her. They don't know how Great and Powerful she really is.