Parallel worlds must have been a reasonably familiar concept to audiences in 1970 (Star Trek had done its famous Mirror, Mirror episode in 1967, although in that case the inhabitants of the mirror universe all had facial hair, whereas here it’s “our” versions that are hirsute). Accordingly, it doesn’t take ages for the Doctor to work out that he’s travelled sideways in time, to what looks like an Orwellian alternative: posters proclaiming ‘Unity is Strength’ with Big Brother looking stern. If that wasn’t clue enough in rapid succession we get Benton shooting at the Doctor, a dark-haired Liz holding him at gunpoint, and a clean-shaven Brigadier modelling an eyepatch.

The parallel world is entirely lacking a sense of humour. It’s like the Doctor has been dropped into an Eric Saward script 14 years early. No-one smiles, the wit is confined to grim jokes like, ‘You won’t feel the bullets when we shoot you’, even Greg and Petra – whose relationship has been thawing back in our universe, remain on frosty, surname only terms here. Genial Sir Keith is dead (I think it’s weird given the Royal Family were executed in 1943 that knighthoods still exist in the parallel world). The Doctor’s good intentions, wit and charm count for nothing. The script makes it clear that he has no place in this version of reality. All its inhabitants want to do is kill him. It’s like the first episode of The War Games grinding the Doctor up in its grim, unrelenting military procedure. Suddenly, a story that looked like it was going to re-do Fury from the Deep has become something else entirely.

The design work is really good at selling the parallel universe: not just the details of the posters, but the plain, utilitarian costumes (the Brigade Leader looks like a member of a South American junta). Even Greg Sutton is wearing a sombre business suit. And the performances are really very good as well: the Brigade Leader has none of the Brigadier’s sparkle; Benton is a thug; Section Leader Shaw is permanently poker-faced and paranoid. Only Professor Stahlman seems relatively unchanged, as arrogant and pig-headed in this universe as the last one. This is suddenly a much more frightening story.

Next episode: Inferno – Episode 4