So far this is possibly the most X-Filesy Doctor Who, with its dodgy government contracts, colourful locals and something nasty buried below the Earth infecting people. There’s a strand of thought that UNIT stories are all like this, but usually they’re providing security for something, or acting as official observers, rather than the Brigadier, in civvies, having a nosey. Courtney looks great in his flat cap (I’m less sold on Pertwee’s dog blanket), but no-one can hold a candle to the incredible cuteness of Katy Manning in her miner’s gear, hat askew.

This episode is all about thickening the plot, with the mine apparently sabotaged, Global Chemicals unwilling to lift a finger to help the trapped miners, and a disembodied voice directing Stevens’ actions. This does lead to some meandering, as the Doctor and the Professor, who immediately get on, join forces to break into Global Chemicals, only to discover that Stevens has been telling the truth: the vital cutting equipment to free the miners is no longer there. However, Michael E Briant’s direction is dynamic enough that the sequence is worthwhile just for Professor Jones’ protest march, and the rapid, panicky camerawork as the Doctor is caged inside the facility.

All this is preamble for the Doctor’s own descent into the mines, where he finds Jo, and they discover a putrid lake of fluorescent green ooze crawling with giant maggots. As more of the disgusting creatures explode from the walls and crawl towards the Doctor and Jo snapping their maggoty teeth threateningly, one of Doctor Who’s iconic moments takes shape. As a metaphor for political and corporate corruption, pollution and the defilement of nature it’s hard to think of a better monster. Everything is working together to make this impactful. The contrast with The Time Monster couldn’t be greater.

Next episode: The Green Death – Episode Three