The episode has two jobs to do. First, it needs to resolve the mysteries raised last week, and second, and more importantly, it needs to set up Vicki as the new companion. So as to avoid interfering with the real story – the introduction of the new regular – the mysteries are handled efficiently and effectively, and without any undue fuss. Once again the Doctor plays investigator – a role he’s played with the Tribe of Gum, in the city of Millennius, and on the Sense-Sphere. Having discovered Bennett’s secret escape hatch, and already aware that ‘Koquillion’ is just a ceremonial costume, it’s not much of a stretch to deduce what’s happened.

Although it’s a slight mystery, it’s no more so than any number of TV “mystery of the week” shows. Elementary does stuff like this for 22 episodes a year. And the denouement is handled brilliantly. Waiting, fearlessly, in the Dido People’s Hall of Judgement, the Doctor unmasks Bennett as a grubby little murderer: a man who’s willing to commit genocide to cover up his crimes. Whitaker’s scripts repeatedly emphasise that the worst of humanity is as bad as any space monster – the politicians on Vulcan; Salamander; General Carrington. The much-praised joke that what looks like a man in a monster suit does, indeed, turn out to be a man in a monster suit is only half the story. Whitaker’s real point is that what looks human can be monstrous, and what seems monstrous can turn out to be perfectly innocent – Sandy the Sand Beast is a harmless vegetarian (despite Barry’s rather neat attempts to make it look menacing as it lurks in the cave mouth). The theme is also implicit in some of Vicki’s dialogue: first when she disbelieves Barbara and Ian’s assertion that the Doctor is ‘from a different age, a different planet altogether’, and then when she dismisses the TARDIS as ‘that old box’ which the Doctor chides her ‘can travel anywhere and everywhere’. Ultimately, Whitaker suggests, you should judge on actions, not appearances.

This idea falls down a bit, though, because the moment the Doctor walks through the space ship door and he and Vicki clap eyes on each other, it’s love at first sight. He’s clearly enchanted. She says ‘as soon as he walked in, I felt that you could trust him.’ He’s holding hands with her within minutes, and finally he asks her to come with him to travel anywhere, regardless of space and time: ‘If you like adventure, my dear, I can promise you an abundance of it.’ This is astonishing: it’s the only time Hartnell’s Doctor ever does this – he gives Ian and Barbara no choice, and all his later companions are stowaways or barge on board. He’s fallen for her as hard as he will with Jo, or the second Romana, or Rose. RTD must have been channeling this moment for all his companion seduction sequences.

O’Brien is impressive at bouncing between teenage anger at Barbara, and enchantment with the Doctor: from petulance to giggling silliness, much like the Doctor himself. I do wonder at what stage of production on the previous story it had been decided that Jenny wouldn’t be the new companion, because there’s a lot of her and Dortmun (another gruff character who’s lost the use of his legs) in Vicki’s relationship with Bennett. I guess at a relatively early stage, because the Doctor never gets any chance to bond with Jenny as he does with Vicki. Desperate Measures impresses because beyond a neat resolution to the mystery of Koquillion it establishes one of the best Doctor and companion pairings in the entire series, one that’s based not on a grandfather / granddaughter familial bond, but what looks very much like a genuine meeting of soul mates.

Next episode: The Slave Traders