The final episode switches, for a chunk of its length, into a base under siege, with the Doctor and Sarah Jane defending the last humans against the Wirrn swarm. It’s practically the first opportunity we’ve had to see the two of them together. The partnership works well, with slightly less of the sarcasm Sarah demonstrates towards the third Doctor, and not much of Pertwee’s charm. Instead, the Doctor winds Sarah up to rile her into completing her mission, laughs at her indignation, and then half-reassures her they’re safe (although he seems a bit less confident on that point than I suspect Pertwee would have played it).

After three episodes of being suffocated, frozen and largely being a bystander, Sarah Jane comes up with the goods here, not only suggesting a solution to the Ark’s power problem (use the space shuttle), but doing the hard work of getting the power cable from the shuttle to the cryogenic chamber. Instead, it’s Harry who stands about not doing much of use, although his hopelessly old school attempts to buck up Sarah’s spirits make the two of them a great double act even over the radio.

I like the end of the story, which relies on two instances of human self-sacrifice to defeat the all-consuming swarm of Wirrn. Rogin, citing the space technicians’ union in a very 1970s joke, insists on being the one to launch the rocket containing the Wirrn swarm out into deep space. Then the hint of Noah that survives destroys the ship. It’s not difficult to take a message that Rogin, coded as a working class character on the Ark, is the one who saves the day against the ‘insect-like conformity’ of the Wirrn and the similarly regimented future humans in their frozen honeycomb. But I think it’s equally just a punch-the-air moment for the indomitable human spirit the Doctor described.

And then, instead of the regulars slipping quietly away in the TARDIS we finish with a Hartnell style segue into the next serial as they teleport down to the scorched Earth. Unintentionally, it finishes the serial with a note of unresolved suspense: who will they find down there, and what will be left of them? While hardly the quintessential fourth Doctor story – Tom Baker’s performance is still a little careful, the script still too overshadowed by five years of Jon Pertwee – I’d argue this is seminal. The space gothic; the emphasis on the horror elements; the obvious debt to Hammer films are all here. It sets the scene for the next three years.

Next episode: The Sontaran Experiment