If you squint, The Green Death belongs in an alternative universe where the Season Seven approach wasn’t so comprehensively binned by Terror of the Autons. The third Doctor is possibly closer to Derrick Sherwin’s concept than in any other Pertwee story: dressing up and doing funny voices. UNIT is more central than they have been in anything since Season Eight. There’s a clear, contemporary ecological message, and, no alien villains for the first time since Inferno (plus, obviously, green slime). After three series increasingly focused on getting the show back into outer space, and a tenth anniversary season that’s focused on bringing back Troughton and recreating a Hartnell epic, this is the one for viewers nostalgic for 1970.

It also feels like a bit of a last hurrah for the Fam Letts and Dicks created three years previously. The Brigadier is back in uniform, confidently predicting that blowing up the mine will end the maggot menace. Captain Yates finally gets something interesting to do, going undercover in Global Chemicals. Benton menaces a milkman. And Jo gets a last chance to be absurdly impetuous, heading off to nobble a maggot for Professor Jones in a scene dripping with references to her introduction in Terror of the Autons (in both of them she’s compared to a tea lady after ruining an experiment). It’s a tribute to Katy Manning’s performance that the tone is so different – when she met the Doctor she was clumsy because she was naïve and intimidated, whereas this time it’s because she’s falling in love. And she answers back a lot better too, asking Cliff if he wants a cup of arsenic.

The episode’s two set pieces are the confrontation with a slag heap full of bulletproof giant maggots (this super power seems a bit redundant – the sheer number of the disgusting creatures should make them threatening enough), and the Doctor breaking into Global Chemicals to try to secure a sample of their toxic waste. In a roundabout way this leads him to the top floor, where the rich and gravelly-voiced boss is revealed not to be the Master, or some insane billionaire, but a computer. It came as a bit of a shock to me to remember that up until now viewers might very well have expected the Master to turn up directing Stevens through hypnosis, and the news it’s actually a computer is cliffhanger-worthy: the twist is that the baddie isn’t an alien supervillain.

Next episode: The Green Death – Episode Five