Doctor Who: Masters of Earth

The events of The Dalek Invasion of Earth have been used in a number of tie-in stories (including Big Finish audio) before. However, such stories have (at least to my knowledge) always been set in the aftermath of the TV story, based around Earth's attempt to rebuild, and, inevitably, some Daleks turning out still to be around somewhere. This is different, in that it's set a year before the TV story, well into, but not beyond, the time of the occupation.

This, inevitably, leads to a different, and rather unusual, sort of story. Because he knows his earlier self will eventually face and defeat them, the Sixth Doctor has to avoid doing so on this occasion, or even giving them any foreknowledge of who he is.

The story avoids too much duplication of the original by being set in northern Scotland, rather than London and Bedfordshire, but it also has all the trappings of that earlier tale: Robomen, the Slyther, and so on. That the Doctor and Peri spend three quarters of this story on the run from the Daleks adds to the recreation of the atmosphere, and, for the same duration, the story is a largely personal one without world-shaking stakes, painting a bleak picture of the Dalek Occupation.

In the final quarter, we have something that's more akin to a typical Dalek story, although, for bonus points, it does foreshadow an episode of the modern TV series (tangentially, of course, given BF's license at the time). This is, to my mind, rather less strong than the previous parts, which are stronger on atmosphere and individual peril. It's also true that Peri isn't as well used here as she might be, being a fairly generic companion and mostly being threatened.

I think you would have to be familiar with the Dalek stories of the '60s to really enjoy this and get all the references. As well as, of course, be okay with the idea that the Doctor doesn't - indeed can't - save the planet at the end of the tale. For me, however, these features only enhanced the atmosphere and I found it an effective prequel/sequel to DIoE.