‘It’s as though someone’s been rummaging around in my memories.’ This really does feel like a Hartnell throwback, with a romantic sub-plot for Tegan, and the Doctor, on the moral high ground, confronting Striker and the Eternals’ dependence on ephemeral minds to fuel their own existence. The episode isn’t packed with incident, but it’s full of ideas and character – the opposite of the “and then this happened and then this happened” approach that’s often the nature of scripts during this period.

And by making the fifth Doctor so unreservedly Hartnellish, Clegg overcomes many of the limitations some other writers projected onto the character. Here, he isn’t “vulnerable” or “unsure of himself”, he’s deducing, he’s confronting, he’s cleverly looking for anachronisms or things that don’t add up – like Critas the Greek’s ring. His is the voice of moral authority against Striker’s sensual decadence. I think this is the strongest showing for Davison so far – it has the passion and energy of Snakedance, but with greater credibility.

Against him, Striker (an astonishingly controlled performance by Keith Barron) looks craven, arrogance and emptiness all mixed up. Striker’s dialogue is superb: he talks of ’the endless wastes of eternity’ and suggests that only novelty and risk make infinity bearable. Like Mawdryn, he is a being tormented by immortality – a theme the show keeps returning to during its anniversary year. His desires are left vague, which only makes him intriguing. The prize in this race is Enlightenment: ‘The wisdom which knows all things and which will enable me to achieve what I desire most. Do not ask what it is. I will not tell you.’

Tegan and Turlough, having been shunted out of Terminus, get much more rewarding roles in this. Tegan is the reluctant object of affection. Marriner’s fascination for her is expressed through the hoariest of cliches (presumably he’s had to find the words raiding the minds of the crew, raised on Victorian melodramas): ‘You’re not like any ephemeral I’ve met before.’ Janet Fielding’s performance this year has been much more subdued than in Season 19, and the result is she makes Tegan sound genuinely frustrated by Marriner’s beliefs, rather than just angry at him. Turlough, meanwhile, is chafing against the Black Guardian’s chains. Strickson continues to play “shifty” brilliantly, like when he’s presented with the key to the rum locker and a chance to tip the game in his favour. The cliffhanger, when he’s finally driven to self-destruction rather than a lifetime of torment by the Black Guardian, is shocking.

Next episode: Enlightenment – Part Three