It took me a lot of listens to get my head around this one, which was due to a combination of things, primarily being very distracted by work stress. Also I don't think there was enough variation in Frazer Hines' voices, so getting a sense of multiple characters being there was not easy. Finally, though I get the premise (at last), I don't think that premise works so well on audio.

The crew lands in Switzerland at some point in the future. I got no sense of Switzerland from the audio, it was just some city... Anyway, the city has two versions existing simultaneously, one where things have fallen to ruin, one where people haven't caved to despair. This is the result of some experiment, we find out at the end. The shifts between the two versions might have looked good on screen, but I never got much feel for it in the audio. The Hawkers, big metal birds that envelop people and turn box-shaped might look great onscreen, but conceptually they make no sense whatsoever, and what explanation we get for them is weak.

There's a mildly interesting idea here about the world you live in being the one you decide to live in, but the execution leaves something to be desired.