Twilight finds the mirror-portal behind Princess Celestia's throne after being left behind at the wedding rehearsal. But instead of Canterlot High, it sends herself, Rainbow Dash and Spike to Terra before the Unification Wars.
Fluttershy and Rainbow Dash get sent to the world of Left 4 Dead. Surrounded by zombies that want to make them into their lunch, they must team up with a group of survivors in order to get home.