The edge of a new face in the opening credits for the first time in five years is slightly offset by the fact that the time tunnel/diamond logo sequence is otherwise just a refinement of the Pertwee titles. It sets the scene for an episode that balances the shock of the new with plenty of efforts to reassure viewers that this is still the same show. In that respect it’s a contrast to Spearhead from Space, which went out of its way to look and feel different (and for obvious reasons couldn’t even include a recap of the second Doctor’s regeneration, whereas this opens with a reprise of the final moment of Planet of the Spiders).

There are obviously good practical reasons for relying on the repertory of semi-regulars. Pertwee had played the Doctor longer than anyone else, and so providing lots of familiar faces around the new Doctor helped to de-risk the transition: seeing the changeover through the Brigadier and Sarah Jane’s eyes allows us to accept the new man just as they do. Plus it’s a chance for a new actor to test his performance and settle without having to carry the whole episode. Nicholas Courtney gets a slightly bigger role than he had through most of Season 11’s UNIT episodes. The Brigadier initially seems unfazed by the regeneration there’s a lovely moment later where he betrays some repressed grief: ‘Miss having him about.’ And Sarah Jane gets to practice her journalist skills again as she meets the past and present members of Think Tank. These are all characterised with Terrance Dicks’ typical efficiency – the eccentric Professor; the cold and sneering director and her fawningly camp assistant.

But despite a fairly standard UNIT story set-up, it’s not all same old same old. Dr Harry Sullivan arrives, with a condescending old-school “doctor knows best” manner that doesn’t survive first contact with the definite article. Ian Marter doesn’t get a huge amount to do in the episode, and it’s not yet clear that he isn’t just a new Mike Yates, but he gets to play the best scene. And recording the whole thing on videotape gives this a crisp, OB immediacy that makes it subtly but palpably different from all those Pertwee location films.

All this is preamble, though, to the main event: the arrival of Tom Baker. If everything else in the episode is set up to reassure viewers about the transformation, that’s because the fourth Doctor is so disconcerting, and not at all reassuring. Like very early Troughton (The Power of the Daleks and The Highlanders), the new Doctor is unreliable and wolfish. Note the helpless look Sarah gives the Brigadier when he tells them he’s off. While the third Doctor made a couple of token attempts to flee the Earth early on, this version actually has the means to abandon the Brigadier, UNIT, Sarah Jane, and us. But beyond even the second Doctor, he seems to take ghoulish delight in the terrors of the universe, relishing the grisly poetry of an enemy that ‘kills a man as casually as it crushes a dandelion’.

Even trying to put aside nearly 50 years of hindsight, Baker’s performance is astonishing. It goes beyond K’anpo’s promised ‘erratic’ to include a hint of otherness, elfish without being ethereal. The way Baker plays his first scene with Sladen contains elements of his turn as Blackadder‘s Captain Redbeard Rum a decade later – a total disregard for norms which would come across as arrogance if the Doctor weren’t also so strange, as baffled by humans (his face as he struggles to explain how he feels to Harry) as they are by him. Dicks’ includes of a couple of nods to the previous Doctor (the TARDIS key is in his shoes, just like in Spearhead from Space; the mirror scene with the joke about Pertwee’s nose, and a comment about the ears that will crop up again in Rose) and the new costume is a tramp version of Pertwee’s (corduroy instead of velvet; cardigan not waistcoat; scruffy necktie rather than cravat) in just the way Troughton’s was hobo Hartnell. But Baker defies straight comparison with his predecessors. From the off, he has become a new Doctor paradigm.

Next episode: Robot – Part Two