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 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.
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.