In practice the android invasion is confined to about two rooms in the space centre. This certainly has all the elements of a climactic episode – stuff that in theory should be fairly solid. Tom Baker playing an evil duplicate of himself and the two versions getting to fight, with the benefit of seven years of editing and effects evolution since Barry Letts last had a go at directing a scene like this in The Enemy of the World. Sarah Jane finally getting back to Britain in 1980, and being reunited with her friends Harry and Benton and (checks notes) Colonel Faraday.

This is quite slow, but it definitely has more purpose and tension than Part Three. Without showing off about it, Baker has fun with the dual role, injecting little tells to indicate which is which. The slight stagger as the real Doctor exits a hijacked pod clues us this is the real deal. The android version has cold, unsmiling eyes. But elsewhere Baker introduces the unwelcome ‘Ask me why…’ prompt which always puts my teeth on edge a bit (yes the companion has to elicit exposition, but we don’t really need it pointing out).

Sadly the rest of this is rubbish, and full of the kind of slapdash plotting that Robert Holmes really should have corrected. The Doctor just happens to have a robot detector in his pocket when it’s convenient to shortcut the story (but we also learn that the androids can’t recognise each other from the human originals, which seems like a significant design flaw). Having kept Sarah Jane alive to test the virus on which his entire plan hinges, Styggron not only didn’t bother to check she’d been administered it, but here we learn he didn’t even bother to check for any results. The robot Doctor tells Crayford the truth about the Kraal plans even though it doesn’t need to and it leads to Styggron’s death. And then there’s the idea that Crayford never had a missing eye and had never thought to look under his eyepatch, which on paper seems like the stupidest flaw of all, but taken alongside all the other issues just makes Styggron look supremely arrogant in his confidence about his brainwashing of Crayford.

I think this is weak stuff, but it’s not entirely unlikeable. The first half is largely very effective, and like Planet of Evil it’s only after the halfway point that things start to go badly off the rails. It’s lovely to see Ian Marter and John Levene again, even if the joy of their reunion is entirely carried by Elisabeth Sladen’s effort rather than any material she’s been given to work with. I like the ongoing joke of Sarah Jane threatening to stay behind and being charmed by the Doctor (let’s see how that works out for her). But if I didn’t know better, I’d say this was made out of obligation rather than enthusiasm. It has the whiff of fulfilling agreements to Terry Nation for a script a year; to Barry Letts, who was looking for directing work; to Ian Marter, who was regrettably dropped before he’d recorded a standard 26-episode run. I don’t blame Nick Courtney for not turning up.

Next episode: The Brain of Morbius