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.

In general, this is a very funny episode. The Doctor’s confrontation with the Dalek – ‘Oh, my tin friend’ – is completely like the ‘poor pathetic creatures’ bit of The Dalek Invasion of Earth, right down to Hartnell’s delivery. Steven and the Doctor’s final squabbling is great. I bet Spooner was chuckling to himself when he gave Hartnell the line, ‘I happen to be the leader of this expedition and I don’t want to keep repeating myself.’

I love that Sara mocks the futuristic technology of Steven’s time – ‘the Romans still used treadmills’ – but his crude lash-ups tend to be more successful than Sara and the Doctor’s efforts. Steven’s quietly a really useful character. The TARDIS force-field is name-checked. RTD used a similar idea in The Parting of the Ways.

Overall, this does feel like a cheap middle episode – small cast, invisible monsters, existing sets, lots of faffing about on board the stolen Dalek spaceship. Spooner makes up for it with the Chen / Black Dalek bickering and some good comedy and commanding material for Hartnell

Next episode: The Feast of Steven