‘They’re trying to control the world with chemical weapons. Let’s join forces. Fight the real enemy.’ “So, the secret of defeating Fenric is an illegal chess move and a quick chat with the Ancient One. Simples. Now that’s all sorted let’s dry clean our clothes and go for a swim and try not to worry about your grandma turning up on the doorstep of a complete stranger in Streatham.” I bet Kathleen nursed a grudge against that Ace for years for sending her on a wild goose chase.

As broadcast, The Curse of Fenric breaks every rule in the South Park writers’ guide: events taking place don’t seem to bear much relation to anything that has occurred previously, so come across as arbitrary “and then this happens and then this happens” – less a story, more a bunch of stuff that happened. The Ancient One kills all the vampires because the writer needs a way to get rid of them; the Doctor reveals that the Ancient One’s future Earth (which we’ve never seen) is the result of its actions in the present.

This isn’t entirely Ian Briggs’ fault: this is another casualty of this era’s chronically over-written, brutally edited stories. Spoiled over the last 30 years by various extended versions on VHS, DVD and Blu-Ray, it’s a bit of a shock to return to the programme as broadcast and realise quite how much it seems like a synopsis of something better. There’s enough to leave an impression of greatness, and some of the sequences are super (here, the dissolving vampire skeletons, the chutzpah of the reveal that Ace has created her own history, Dinsdale Landen’s grinning Fenric), but if this were the only version of The Curse of Fenric we had, I don’t think its reputation would be as strong.

All that aside, it has a supremely effective horror mood, easily the best the show had done for a dozen years. That the extended versions buck the usual trend and are better than the original, rather than just longer, speaks to the fact that there is tonnes of really good material in it. Pitting the seventh Doctor against a villain that’s even more adept at manipulation is a good move, and it’s neat to answer the mystery of Ace’s time storm and to suggest that Fenric placed her with the Doctor to draw him into a final confrontation.

The weakest link is Ace herself: this was conceived as the season opener, so it makes sense she’s more girlish than in Ghost Light (which was meant to be transmitted after), and this represents a moment of maturity for her. The idea is sound, the execution is – well, being generous it’s not entirely successful. Now, if only they had a writer who could convincingly depict a teenage girl on the brink of emotional and sexual maturity, they might really be onto something.

Next episode: Survival