While UNIT race to stop the Master from launching the stolen Thunderbolt missile, the Doctor has his own challenge: to bring a temporary halt to the alien mind parasite’s increasingly greedy attempts to feed on the evil in Stangmoor Prison. Meanwhile, Mike Yates is strapped to a chair at the launch site, and Jo’s locked in a cell surrounded by sex-starved male prisoners: in context, Mailer’s little air kiss to her is one of the scariest things in the entire series.

This is just one example of some of the really strong character work in the script and performances. One of the great strengths of Inferno was Don Houghton’s soap opera ability to make us invest in the relationships between characters. That’s been slightly less apparent in The Mind of Evil so far, but is very clear here: it’s the episode where the UNIT Family comes into focus. The Doctor and Jo get Pertwee’s first great “moment of charm” as they share some leftover food and water, and the Doctor tells Jo about the time he was locked up in the Tower with Raleigh. This is after the two of them have shushed the Master while they finish a game of draughts (Jo wins).

Incredibly, given the story is setting him up as a suave Bond villain, the Master not only tolerates this, but shows no animosity toward the Doctor when the two of them are working on solving the problem of the mind parasite. ‘What can I do to help?’ he asks, before wishing ‘Good luck to you, Doctor.’ There’s a sense of genuine camaraderie between them (which is, cleverly, useful set up for the next serial). Elsewhere, John Levene shares a lovely scene with Nick Courtney (‘You said yourself, I’ve got a thick skull’) and asks about Mike. Although it’s only really going to last for a handful of stories, the crucial relationships for the next three years are now set.

In between, Timothy Combe finds time for an impressively staged attack on Stangmoor Prison, with Courtney getting a much more rewarding role for the Brigadier than in Terror of the Autons as he impersonates a Cockney lorry driver and coolly inveigles his way into the fortress. And when the Doctor is again assaulted by the mind parasite, he has a vision of a Dalek as well as hearing it. Although 1967’s The Evil of the Daleks was their last story, that was repeated in 1968 and a Dalek turned up in The War Games in 1969 so they’ve never been forgotten. Along with the return of the Police Box in Terror of the Autons, this feels like Letts and Dicks beginning to reintroduce the elements that Sherwin had thrown out in 1969.

Next episode: The Mind of Evil – Episode Six