The first half of the episode is set in the parallel universe, and Don Houghton’s script neatly drops enough hints about this Britain to give us some idea of what happened in 1943. The talk of ‘servants of the state’, ‘party members’ and ‘crackpot free speech groups’ sketch a familiar picture, and Greg’s preordained fate is to be liquidated once the project is complete and his outspokenness outweighs his usefulness.

This is all quite grisly, as is the fairly brutal interrogation sequence – Douglas Camfield (or possibly Barry Letts) directs it as a relentless series of cuts between the Brigade Leader and Shaw barking questions (and trying to implicate Sutton in the process) at a dishevelled and exhausted-looking Doctor as Benton brutally pulls his head back. This is rough treatment of the show’s leading man – we’ve rarely seen the Doctor quite so brutally treated (Hartnell ravaged by the Elders’ life-draining machine in The Savages springs to mind). 

Back in our world for a brief sequence, the Brigadier and Liz talk about the Doctor:

BRIGADIER: Wherever he is or whenever he is, he can look after himself.

LIZ: He’s not indestructible you know

It’s a nice exchange that, taken with the interrogation scenes, remind us that the third Doctor is oddly more vulnerable than his previous self: the second Doctor’s clowning hid his power and intellect; the third is the opposite: his confident swagger hides his vulnerability. Cut off from his TARDIS, deprived of a lot of his knowledge, he’s having to bluff his way through life much more than he used to. No wonder he develops a greed for regaining the things the Time Lords took from him.

Trapped in the parallel world, the Doctor tries to reach out to the closest version of someone familiar: Liz (or Elizabeth, as he formally calls her). She talks about learning the ‘true facts’ and he recognises that she has a scientific mind ‘Whatever they taught you in this bigoted world.’ When she tells the Doctor ‘Everybody talks’ she sounds almost regretful. But she’s no walkover, and still doesn’t believe a word of his alternative universe story. Neither does anyone listen to the Doctor’s warnings about Stahlman: at some point he has worked out it’s the green ooze that infects people (although the audience knew the Doctor hadn’t mentioned it explicitly before). 

Back in our world, Sir Keith is not looking forward to Penetration Zero: ‘There’s something ominous about the whole project. Your friend the Doctor felt it too.’ This pivots this episode into its final few minutes – suddenly everything is overshadowed by the parallel world’s imminent breakthrough. As the Doctor manages to sneak back into the control room, Greg declares, ‘Something’s wrong.’ A timer ticks down; the whine of the drill is deafening. The Brigade Leader paces about nervously. The Doctor yells out his warning of the ‘planet screaming out its rage’ as a voice counts down into the end credits. This is as tense as Doctor Who gets.

Next episode: Inferno – Episode 5