One quarry, one potato-headed alien, and a Doctor with a broken collarbone.
Short Synopsis
The Doctor, Sarah, and Harry land on a ruined Earth and discover that a Sontaran has set up shop, using stranded humans as lab rats in his latest experiment on how best to conquer humanity. Spoiler: the experiment ends badly (for him).
Extended Storyline
Coming straight off The Ark in Space, the TARDIS crew transmat down to Earth to make sure it’s safe for humanity to return. Spoiler: it’s not. Earth looks suspiciously like a Devon quarry, and the only inhabitants are unlucky space travelers being subjected to cruel experiments by Styre, a lone Sontaran with the bedside manner of a traffic warden.
Styre’s “scientific” tests include things like:
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How long can humans last without water?
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How fast do humans die if you bake them in the sun?
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How much can humans squirm before getting bored of screaming?
(It’s basically the alien equivalent of a Year 9 science fair project.)
The Doctor investigates while Sarah Jane gets captured (classic) and Harry tries to be useful but mostly just ends up tripping over rocks. Tom Baker, in his second full story, was already showing off his alien eccentricities, though production had to work around his real-life broken collarbone — which meant stunt doubles and “don’t let Tom actually fall down the quarry, please.”
Eventually, the Doctor confronts Styre, mocks his appearance (pot, kettle, etc.), and challenges him to single combat. Yes, that’s right — the Fourth Doctor decides the best way to deal with a warlike alien is to wrestle him. Luckily, Styre’s suit malfunctions, he keels over, and the Doctor declares victory without actually needing to do much more than stall for time. The Sontaran ship tries to swoop in, but the Doctor sabotages it because he’s clever like that. Humanity: 1, Potato Men: 0.