This is a really enjoyable yarn.
The sets are good, the acting top notch and the script intelligent. There is a sufficient air of suspense as we don't find out why people get sucked into the ground until later. One of the intriguing parts of the story is that these are the last humans; we've seen a number of junctures in human history through the millenia in Doctor Who by this time (e.g. leaving Earth in the Ark and the Ark in Space; a large galactic empire in Frontier and Space; a declining empire in The Mutants). New Who comes back to these "events" later on so it's worth contrasting those visions at a later stage.
The Tractors are a let down as the costumes don't look all that good. But that's one of the few quibbles.
Silence in the face of injustice is complicity with the oppressor - Fellow-Socialists and Fellow-Workers, the existing order of things appears to be breaking down. And it seems that the old order has had its day and all th...
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