The Doctor has messed up. Because of him the entire world is in disarray. So much damage has fallen into the world due to the sins Time Turner committed. He should be dead now, but he isn't and when everypony finds this out they're going to make sure he gets what he deserves.
But he can't die, not just yet. He needs time to try to make things right, to undo the wrongs he has done, and bring the world back to order. And after finding an oddly familiar young mare, his only hope of succeeding, things start taking a turn for the worse. With the world against them, the two will need each other if they want to survive the cruel ruined world of Equestria.