After the previous episode revealed that the Macra are in charge of the colony, this episode sets out to explain why – and it’s a bit garbled, to be honest. The explanation is that the Macra need the human colonists to mine the poison gas they need to survive. But if that’s the case, why are they running the colony like a holiday camp instead of a labour camp? What’s in it for them? It’s not even as if the happy campers are willing collaborators who get to enjoy luxury lifestyles in return for selling out their comrades: they’re in thrall to the Macra, and it seems the miners – who are the ones ensuring the Macra get their gas – who are the rebellious dissidents.

Black’s obvious dislike for societies built on exploiting the workers clearly comes across – so perhaps it’s a dig at the petty bourgeoise turning a blind eye to terrible conditions of the proletariat – who are working to serve the capitalists. I suspect there’s some sort of Marxist parable going on, and if so it’s amusing that the Macra controller speaks with a plummy, RP accent.

The best bits of the episode are the Doctor’s inductive reasoning that leads him to correctly identify the composition of the poison gas (he scores his efforts 11/10). It’s a scene that’s indirectly mined by other actors – Troughton’s bag of jelly babies, and his habit of chalking formulas on the wall feel like they’re picked up by the fourth and twelfth Doctors respectively.

The other great moments are Jamie’s exploration of the forbidden areas of the mines – dark and shrouded in cobwebs, and ideal for the Macra to loom threateningly out of the shadows. I imagine these bits sent a lot of kids to bed with nightmares.

Other things:

• The mine controller Officia is played by John Harvey – Professor Brett in Black’s The War Machines
• Jamie and Polly are briefly paired up in the mines – it’s one of the only times the two of them get to interact without Ben (in the last couple of stories we’ve had Ben and Jamie, and Ben and Polly, but not really this pairing)
• The scene where the Doctor works out the formula from the computer equipment feels like it’s fulfilling the Sherlock Holmes element of the original character brief

Next episode: The Macra Terror – Episode 4