Barbara is back, and it looks like she’s been making the most of the spaceship’s tanning salon during the previous fortnight. She doesn’t mess about – while the Doctor and Ian are lost in the tunnels under the city, she organises the rescue mission, commandeers the Sensorites’ telepathic transmitters and heads into the caves herself. This all adds a sense of urgency and purpose to this episode that was sorely missing from the last one.

Ian and the Doctor get a bit less to do this week – they’re almost a comedy double act (as they were in The Keys of Marinus), with Ian nervously backing away from the insane spacemen as the oblivious Doctor witters on about collecting evidence and chiding Ian: ‘Don’t interrupt, young boy. It’s most irritating.’

They’re confronted by a group of astronauts clearly inspired by stories of isolated jungle troops still fighting the Second World War into the 1950s (they repeatedly talk about ‘the war with the Sensorites’). Newman’s insight into the military means these scenes have a peculiar poignancy and authenticity to them: the commander, still keeping military discipline and talking in clipped tones about ‘the men’ when he’s basically got two desperate followers. It’s a fitting end to an adventure that has repeatedly shown the villains to be those incapable of seeing things from another’s point of view, or opening their minds to new possibilities. This is in contrast to the heroes, who have a more questioning, less prejudiced perspective, willing to believe the evidence rather than their own assumptions.

Carole Ann Ford also gets a slightly more rewarding part this week, making use of her newfound psychic abilities, and even getting the best line – a description of her home planet that’s so hauntingly beautiful it was lifted verbatim by Russell T. Davies. There’s also another hint that she’s growing apart from her grandfather: while he talks about going home as something to do in the vague and distant future, whereas Susan seems to be tiring of being a wanderer in the fourth dimension: ‘Sometimes I feel I’d like to belong somewhere.’ I imagine at this point decisions were being made about the cast for the second production block, and writing Susan out. Which means that her departure is more heavily signposted than any other companion in the classic series.

This all contributes to my feeling that The Sensorites is a tipping point. There are references back to the original premise of the show, from the ‘mild curiosity in a junkyard’ scene in Strangers in Space, and the moment in The Unwilling Warriors where Barbara explains to Carol that she and Ian are trying to get home. Here, Susan tells the First Elder, ‘We’ll all go home some day’ before her final, wistful conversation with the Doctor about settling down. But this is balanced out by the Doctor’s obvious enjoyment of his travels, and his offhanded attitude to going home: ‘This old ship of mine seems to be an aimless thing. However, we don’t worry about it, do we?’. And even Susan isn’t in a hurry to stop travelling – the First Elder looks into her mind and perceives, ‘You wish to see your home again, and yet there is a part of you which calls for adventure. A wanderlust.’ Maybe that odd little scene at the start of Strangers in Space is less a recap, and more a reformatting of the series’ ambition: ‘It all started out as a mild curiosity in a junkyard, and now it’s turned out to be quite a great spirit of adventure.’

The punchline to all this talk of wandering and going home is the Doctor’s outraged reaction to Ian’s implication that he hasn’t got a clue what he’s doing, and his threat to put Ian and Barbara off the Ship at its next destination. But by the end of The Sensorites – which, even more than The Aztecs, focuses on the Doctor and relegates the rest of the TARDIS crew to being his companions – it’s become perfectly possible to see how this series can continue even if Susan settles down, and Ian and Barbara get home. So long as the Doctor and his old Ship are content to aimlessly travel the cosmos, there’s no reason why they can’t keep going forever. The mild curiosity in a junkyard was Ian and Barbara’s. But the great spirit of adventure is the Doctor’s. This is his series now.

Next episode: A Land of Fear