My favourite thing about this story is that the monster isn’t just green, it’s a bubbling lump of purest green. It’s also treated quite straightforwardly as an actively malevolent entity, which started off moving about Fang Rock cautiously, understanding the lie of the land, the defensive capabilities of the locals, and their biology (it conducts a post mortem on Ben, which makes it a sort of outer-space Litefoot). It ‘contrived to isolate us’ and now it’s waiting to strike. Before the night ends, every human being on Fang Rock might be dead.

Maybe it’s because the monster is moving so cautiously that the Doctor’s also operating more subtly than usual. Fair enough, he does burst in on shipwrecked gentlemen Skinsale and Palmerdale having the upper-class equivalent of fisticuffs (lots of threats of trashed reputations and courtrooms) to declare that the lighthouse is under attack. But when Leela suggests telling them the truth, the Doctor retorts, ‘They’ll think we’re mad if we start talking about creatures from outer space.’ That doesn’t normally bother him, but he seems more than usually disturbed by whatever is threatening them. It’s truly chilling when he admits to Leela that he lied to Vince that the undead Ben was merely an effect of electricity: ‘I don’t know what the truth is, yet.’

This is all brilliantly effective. The injection of new characters all efficiently established by Terrance Dicks in best Target Books style – the vile, greedy Palmerdale, the genially dangerous Skinsale, prim and judgemental Adelaide (evidently disapproving when the Doctor puts his feet on the table), and Harker, the worm that turned – doesn’t open the story up, it just makes the lighthouse seem even more claustrophobic. It might look superficially like a sequel to The Talons of Weng-Chiang, but it’s a million miles from that story’s everything-and-the-kitchen-sink indulgence. This is taut, pared back, and as ruthless as the Rutan.

Next episode: Horror of Fang Rock – Part Three