This one, I think, tends to be a little bit undervalued in comparison to some of the other Philip Hinchliffe masterpieces. It lacks the conscious significance of Genesis of the Daleks or The Deadly Assassin, or the Hammer Gothic flair of Pyramids of Mars. And none of those has the reputation of the Skarasen to contend with. But it’s the most solidly constructed and brilliantly executed fourth Doctor serial to date. The third episode has no sag, and twists the story in a direction we probably didn’t see coming – it’s not often the cliffhanger involves the baddies flying away.

Douglas Camfield was the ideal choice for this: he brings the same eye for creepy details and ability to create convincing military manoeuvres that made The Web of Fear so impactful. The scenes of soldiers searching the woods for the Zygon that killed Angus are supremely eerie, mixing handheld shots, glimpses of the monster flitting through the trees, and Sister Lamont slowly emerging from the undergrowth to club down one of the men. Backed by Geoffrey Burgon’s great haunting score, which has hints of his work on A Ghost Story for Christmas: Treasure of Abbot Thomas, it’s as good as anything British horror films were producing in the mid-1970s.

And the studio work, while inevitably less dynamic, is compelling. I adore the scene of the Doctor, Sarah and the Brigadier meeting the Duke: the Doctor provocatively mocking, the Brigadier patiently respectful, the Duke courteously dismissive and furious. The Doctor getting a laugh from an impression of the Duke’s pompous claim that, ‘My family has served this country for seven centuries’ is a bit odd, but sort of works as a nod to the idea that the “Duke” is actually just an impression (or maybe the Doctor is just amused at someone’s esteemed family being younger than himself).

Much of this feels like the edited highlights of Doctor Who and the Silurians (creatures from the depths menacing modern day installations; attacks in a barn and a cottage; the chase of a wounded creature across the moors; a summoning device that can summon a huge monster; a trip to London), but the Arthur C Clarke’s Mysterious World overlay (‘The. Loch. Ness. Monster.’), and the effectiveness of the Zygons themselves make this distinctive.

Next episode: Terror of the Zygons – Part Four