‘We’re all basically primeval slime with ideas above its station.’ A parable of evolution where the Deciders have been frozen in indecision for four thousand generations, repeating the same futile pre-launch procedures without ever pressing the button. Their refusal to adapt to change is finally overturned at this crisis point, as the rest of the Alzarian ecosystem rises up to cast them out. If this were made now, there would be an overt Green commentary, with the human race being expelled from the planet that birthed it, and which it has abused. I suspect there’s something of this theme lingering round the edges (it would certainly chime with executive producer Barry Letts’ instincts).

This is the first Season 18 story to actually work: it doesn’t feel like Bidmead’s heavy hand has fallen too hard; the SF elements are integrated into an elegiac and effective piece that manages to get its points across without either patronising or baffling the audience (not the case for every story this year). There are hints of Inferno, as the planet cries out its rage and its agents invade the heart of human control to infect their less primordial cousins, only to be fought off with fire extinguishers. The explanation of what’s going on comes in a few neatly judged revelations and clarifications rather than a stream of bafflegab exposition.

Some of the images linger: the Doctor using K9’s head as a sort of tribal mask; Varsh being dragged under a closing door; Romana linking hands with a Marshman. There are a few great lines: the Doctor is ‘not a man to side with chaos’, which is mildly debateable but ties in with his rejection of the Black Guardian, and with developments yet to come this season. The possessed Romana reaching out to get her hands round the Doctor’s throat could almost be a meta commentary on Lalla Ward’s contemporary relationship with Tom Baker (in the Making Of documentary George Baker talks about how relations were so broken that they would retreat to opposite ends of the rehearsal room and yell at each other). This isn’t in the first rank of Doctor Who stories, it hasn’t got the panache for that, but it’s as creditable a meat and potatoes serial as you can get.

Next episode: State of Decay