While visiting Canterlot High, the Doctor receives a message from Madam Vastra in Victorian London, about a mysterious girl who's appeared, claiming to know the Doctor. In a rush, he grabs Sunset and Adagio, and rushes to the Paternoster Gang in 1894.
When he and his companions arrive, he discovers that, somehow, the human version of Twilight Sparkle how somehow come back from the dead.
But, that's not the only problem, as it seems that parts of the future are leaking into the past.
With the help of his three companions, and the Paternoster Gang, he must figure out what is happening, and how Twilight has comeback.

'Alternate Timeline' tag for slight changes in the timeline.