For a story that more or less resets by the end, taking place entirely in an alternate reality, this is truly affecting and harrowing in parts. In particular when a kind immigrant family is taken away by soldiers and Wilf reacts with “it’s happening again.” The show is extremely confident at this point, weaving in continuity galore without fear of alienating viewers - fully aware most people watching have done all the required viewing.

Billie Piper seems a bit alien in all this - from a totally different era. She’s brilliant but Catherine Tate continues to steal the show in this Doctor lite story, which sees her taking on the entire dramatic weight of the plot.

If I had any criticism is that it is a little too continuity heavy - relying on plot points gone by. But this is a nitpick as Doctor Who rarely enters this “what if?” territory, so it serves as an interesting flavour.