It’s hardly wall-to-wall action, instead the episode has a steady-as-she-goes pace that brings in the serial to a satisfactory climax. The front half builds the tension as UNIT wait for the Zygons to make their move – having escaped Loch Ness the ship (and the Skarasen) are heading south. Meanwhile, the Doctor manages to get the truth out of Broton: the Zygon refugee fleet is on its way to Earth, and before it arrives Broton’s crew plan to take over the planet and Zygorform it with human slave labour.

I like how much effort has gone into ensuring this doesn’t try for anything over-ambitious or unachievable. There’s no attempt to show the Skarasen rampaging through a model village version of London – lessons have been learnt from Invasion of the Dinosaurs. The Zygon spaceship lands in a disused quarry near Brentford rather than Trafalgar Square, meaning that, for once, a quarry appears as itself. The Zygons are largely kept in subdued lighting in their murky spaceship, or in a cellar, to avoid subjecting the costumes to too much scrutiny. Much of the incidental action is described rather than shown – reports of the Skarasen racing through the North Sea, or the Zygon ship flying over Leicester.

The character work is strong. Sarah Jane gets to do some journalistic investigation and come up with the answer for where Broton is going (Harry’s implication that she’s looking for some headline-grabbing scandal is very unfair). As they head into Broton’s lair, she cracks a joke that gets a smile and an appreciative look from the Doctor, and she gets a nice moment with Benton that implies a closer friendship than we really saw during Season 11. This is her best story since Planet of the Spiders.

The Brigadier also gets his Robot wish fulfilled when he guns down Broton: ‘Just once I’d like to meet an alien menace that wasn’t immune to bullets.’ It’s a shame this didn’t air as the last story of Season 12 as this feels like pay-off. Only Harry gets a bit short changed, reduced to tagging along with Sarah and the Brigadier, and barely getting a goodbye scene (the Doctor just pulls a face when Harry says he’s staying with the Brigadier).

This is very much a last hurrah for UNIT as it was originally conceived and developed. Nicholas Courtney won’t appear again for another 180-plus episodes, so there’s a neat sense of full circle as this comes back to the banks of the Thames in London, scene of the first UNIT serial. It’s also nice synchronicity that Douglas Camfield, who directed The Invasion, directs this. There’s nothing in Terror of the Zygons that we haven’t broadly seen before – most of its content is pretty much covered by Spearhead from Space and Doctor Who and the Silurians – but this is so well made that it becomes pretty much the last word on the UNIT: doing more like this would have been pointless.

There are better Doctor Who stories than Terror of the Zygons. It doesn’t have the scale of insane ambition of some of the Pertwee UNIT episodes which means it succeeds more than something like The Claws of Axos, but only by trying for less. Its biggest reach, the Skarasen, slightly exceeds its grasp (although not by that much – the Rancor in Return of the Jedi, made eight years later with a vastly bigger budget, isn’t that much more convincing). The Doctor’s abandonment of Broton’s crew to their deaths is cold: exactly the kind of calculated sacrifice the old Doctor criticised the Brigadier for (there’s even a nod to this, I think, in the line, ‘Was that bang big enough for you, Brigadier?’). It isn’t perfect, but it gets more right more completely than any previous Doctor Who story. This is a new benchmark for the show.

Next episode: Planet of Evil