The drama of the cliffhanger is let down by a talky stalemate, broken when the Daleks are attacked by the Visians. Invisible monsters in a jungle will crop up again in 1973’s Planet of the Daleks (also by Terry Nation), which reinforces the idea that it was consciously modelled on this epic. Meanwhile, Chen continues to be a gloriously slippery politician, dressing up his failures as tactical masterstrokes. ‘You make your incompetence sound like an achievement,’ says the Black Dalek perceptively. His continual interruption of the increasingly angry Dalek is brilliant. He is uniquely able to fluster the Daleks, reminding them of the need to secure the Taranium before exterminating the Doctor. ‘Of course, of course,’ snaps the Black Dalek in a tizz.

After last week’s grim horror, this is a marked change in tone. The Doctor and Steven don’t even realise Vyon has been killed until Sara tells them much later in the episode. Instead, they – and Sara – are caught up in a matter transmission experiment that beams them and some mice halfway across the galaxy courtesy of a rather horrible solarising effect that turns Hartnell’s face into a gurning skull. Sadly, though, we’re denied him trampolining through space on the way to the distant destination.

The beginning of this episode is pretty much the grimmest Doctor Who ever got. ‘You animal,’ screams Steven as Kirksen drags Katarina into an airlock and demands that the Spar is diverted to Kembel to seek the help of the Daleks. Faced with this, even Vyon dithers, while the Doctor demands they go back to Kembel. The argument, the chaos, clear in the short surviving clips – Steven rushing desperately back and forth, a furious Doctor slapping the controls, Vyon hunched over, hatchet faced – show that the audience is thrown straight into the drama. But Katarina saves all their skins by sacrificing herself. It’s the most dramatic companion exit so far – although Katarina’s really only been a background character – and it sets the tone for an episode that raises the stakes both for this story, and for what Doctor Who can do. It’s also amusing that the Doctor mourns Katarina – who’s been with him for about a day – almost exactly as much as he later does Adric.

The planet in the title is Desperus, and I think ‘Desperation’ might have been a better title for this episode. Not because it lacks incident – more because right from the off there’s a real sense of mounting hysteria right across the board.

The best thing about this episode is Kevin Stoney’s brilliant performance. Mavic Chen – a virtual anagram of Machiavellian – is brilliantly manipulative, pushing Zephon’s buttons, and winding the Daleks up with his emphasis on ‘eventually’. The way he flits about the sets, a corridor conversation here, a scribbled note there, is fabulous. And Stoney makes the most of the dialogue, relishing stuff like ‘the Embodiment Gris’ with a wry smile. It’s the first time the Daleks have been paired with a human ally – but from now on that’s the norm, probably because it works so well here.

Although Doctor Who has been serialised since the beginning, the last few episodes have really pushed the idea that this is a continuing adventure. Mission to the Unknown established the Dalek threat to invade the Solar System. Then the previous week had Steven stabbed and Katarina stumble on board the TARDIS. The Nightmare Begins follows on from both these events.

This episode has the best title ever. Sadly, it’s the greatest thing about it, because even Cotton’s script isn’t up to gracefully engineering the clumsy set of cast changes insisted on by a petulant producer. As a result, this is largely caught up in the mechanics of plot resolution rather than the character comedy of the last few weeks.

The histrionics within Priam’s family continue as Cressida and Paris go at each other, complete with some witty and biting puns – Paris accuses Cassandra of ‘galloping religious mania’ (because they worship horses), and Priam refers to Paris’s ‘flaccid façade’. In general, there are a lot of fairly risqué jokes here, including the Doctor worrying about the Trojan Horse’s fetlocks, making Odysseus ‘as worried as a Bacchanate at her first orgy’.