As punishment for her crimes, the Elements have cursed Sunset Shimmer to do favors for anyone who asks. Lucky her. And then there's that other Twilight Sparkle she keeps running into...
Moondancer conned her neighbor, Trixie, into giving her some solitude when she was a filly, and now she tries to make amends by applying Friendship Theory.
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.
As graduation draws near, magic begins to awaken in the human world, drawing out old rivals and opening the doors to new adventures. It's up to Sunset and her friends to keep Canterlot safe, all while dealing with their looming futures.
After developing a new portal that can explore other dimensions, Twilight and Sunset are pulled into an adventure, where they meet even more doppelgängers of themselves. Together they have to save the day, and learn about each other in the process.
Twilight has been asking Sunset for assistance with experiments quite a bit as of late. As time goes by, Sunset starts to question whether these experiments were purely for scientific purposes... or something deeper?