While a stressful headache a lot of the time, being the ruling princess of Equestria does have some perks. One of those being, if Twilight Sparkle wants to celebrate her birthday by inviting all of her friends, she can do so, even if "all" includes those living in a whole alternate reality. Sure, some work will need to be done to make sure that everyone knows who's actually a pony and who's a temporarily-transformed human, and a few of her other friends will need to be informed that the mirror portal exists shortly before being sworn to secrecy about it, but she can make it work.
Agreeing to come to a party in the magic horse dimension had been a no-brainer for Rainbow Dash, and she's ready to party hard, hopefully while having a opportunity to test these wings out in the process, but when she arrives, she notices someone.
Applejack. The other Applejack. On the arm--foreleg?--of the last person she expected.
It turns out there's somebody else at the party experiencing the same shock. Or rather, somepony.
Maybe together they can figure out how they feel about it.
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