A woman by the name of Kiara Malise arrives in Canterlot, enrolling in it's High School. Devoid of social contact, she's only seen wandering the halls and in the library. But she frequently just disappears.
When a student turns up dead and strange dimensional rifts begin appearing around all of Canterlot, Sunset Shimmer phones the only person she knows can help.
The Doctor returns to Canterlot High, with a new face, to expose a devastating secret.

Alternate Universe tag for various differences in the timeline. Set some time after Rainbow Rocks, but before Friendship Games.