The scale of this serial keeps increasing, as events begin to escalate. As well as Dr Quinn’s cottage, this episode features the biggest action sequence the show has attempted: a cordon of UNIT soldiers sweeps across the moor in a ‘man’ hunt for the escaped Silurian, as a helicopter flies overhead, and the Doctor and Brigadier drive along the ridge observing it all. And all this is just the middle part of the episode, passing without much comment. A year ago, Douglas Camfield’s set pieces were battles between small squads of soldiers and monsters in the confines of Covent Garden or the IE factory. This scale feels like a real difference in Season Seven: the scope of what’s achievable on the budget opening up just as the show’s fictional scope is closing down.

Pleasingly, Malcolm Hulke’s script doesn’t fetishise the military elements. The real meat of the episode are the Doctor’s two visits to Dr Quinn’s cottage, where the the injured Silurian is hiding out – or rather, being held prisoner until Quinn’s associates reveal more of their secrets to him. The first visit simmers with insinuations, as the Doctor slowly needles Quinn about his odd behaviour and the unusually hot temperature in his house. It’s a brilliant sequence: Pertwee plays it initially with a faux-benevolence, innocently forcing his way into the cottage to admire Quinn’s antique furniture, gradually becoming more pointed and threatening even as he keeps a smile on his face. It’s a scene that on paper looks just like something the old Doctor would do, and the difference is all in the performance: Troughton would have maintained the innocent act right up until the last line; Pertwee plays it more like a snake toying with its prey, both of them knowing who’s in control which oddly makes the whole thing seem more unsettling because we almost feel sorry for Quinn.

A quick trip back to the research centre to confirm his suspicions, and to quiz Miss Dawson about her relationship with Quinn (with the implication that she’s got a crush on the old bachelor) and the Doctor returns to the cottage to confront Quinn. But he’s been killed, and the episode ends with the long-delayed reveal of the Silurian striding into Quinn’s parlour to corner the Doctor. Forget Autons on Ealing high street: this is the new approach in a single shot. The monsters aren’t in space, or out on the streets, they’re here, taking over your front room, and hiding behind the sofa won’t save you.

Next episode: Doctor Who and the Silurians – Episode 4