The centrepiece of the episode, and the story, is the Doctor’s one-to-one with Davros, in which the Doctor compares the Daleks to a deadly virus inimical to all life, and Davros shows his true colours by ranting about the absolute power the Daleks will grant him. The analogy makes is clear that Davros’s only interest is power for its own sake, not with any purpose of doing good. He’d see all life but the Daleks wiped out. This would, clearly, have the side effect of creating peace, but only because there would be no-one left to disagree. It’s echoed in the later scene when Davros talks sneeringly of “democracy” as the ‘creed of cowards’ and states ‘Achievement comes through absolute power, and power through strength.’

It is a justly praised scene because it says so much about Davros and the Daleks, but also about the Doctor. It’s hard to imagine many situations where Pertwee or Troughton’s Doctors would essentially choke another person unless he agreed to their demands, and then threaten to finish them off. That’s what the fourth Doctor does here, when he switches off Davros’s life support.

Partly it shows the desperation of the Doctor’s situation – Davros has already told him ‘You have changed the future of the universe, Doctor’ and if the mission now fails, the Doctor has changed his entire personal history into a string of defeats. But it also hints at a darker edge to the new Doctor largely absent from his previous incarnations. A sense of morality and choice on a cosmic scale that Hinchcliffe and Holmes will come back to. It’s a hard trick to pull off well (when it was tried again in the mid-1980s it fell down because Colin Baker wasn’t pitted against epic existential danger, but some guards on Varos and a fat alien jogging through a field).

By the end of the episode there are several stories all converging on an attempt to destroy the Daleks: the Thal attempt to infiltrate the Dome; the Kaled rebellion, and the Doctor’s attempts to destroy the hatchery. It leads to a couple of great visual moments: the first comes when the Daleks sweep through the trenches. Show from beneath, they loom across the dark horizon. It’s so striking I can believe it inspired the eighth Doctor WW1 Dalek play Dark Eyes. And the cliffhanger finally has a Dalek mutant attacking. We saw at least a glimpse of the Dalek creatures in every 1960s Dalek serial bar The Dalek Invasion of Earth, but to date in the 1970s they’ve been kept offscreen. Here, though, we get a newly-hatched bubbling lump of hate throttling the Doctor, choking the life out of him. Even outside the shell, they’re deadly – as we’ll see again in Resurrection of the Daleks, Twice Upon a Time and Resolution.

Next episode: Genesis of the Daleks – Part Six