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.
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.