Hogwarts School of Witchcraft and Wizardry has protected its own through time and realities. It did so even when wizards could not see the foolishness of their actions. Most of the times, the castle and its inhabitants have been able to stop the nefarious acts of a few, but that is not always the case.
The year is 2038 and in one reality, Hogwarts stands devoid of students. The ghosts and the house-elves keep thing going, but there are no more wizards and witches in the country. They have all disappeared and the few Muggleborns have no one to guide them as they discover their own powers. The Muggles keep living, ignoring the war that has ended the life of all the wizards.
With no one else left to help, Hogwarts decides to fix things. Slowly, one by one, wizards and witches from different times and realities begin the appear. The house elves cheer up. The castle rejoices, but what happens to the people who suddenly find themselves displaced.