‘Something mysterious inside a school: that would be ridiculous, wouldn’t it?’ The series proper launches with an episode that’s largely a mash-up of Aliens of London and School Reunion. Which underestimates how fresh it feels coming off the back of Series Three, which, with episodes like Human Nature and The Sound of Drums, was clearly pitched at an audience two years older than in 2005. And as Doctor Who “matures” with its viewers, it creates space for this, a show much closer in tone to Eccleston than latterday Tennant.

So, we return to the Slitheen. Part of the joy of Aliens of London was the incongruity of seeing the corridors of power populated by farting, giggling aliens. It goes for a similar tone (plus the genius addition of a child Slitheen: the script, rightly, doesn’t dwell on the grisly fate that befell the child whose skin it’s wearing, although I wonder if the real Carl was as camp as the Slitheen version), with the headmaster of Park Vale School giving a flatulent assembly complete with a half-bothered “inspirational” speck to the children. They include Luke and Maria returning from Invasion of the Bane, and the thankful addition of Daniel Anthony as Clyde, the much-needed Han to their Luke and Leia. From the off, this trio works much better than in the New Year’s Day episode, with Luke’s super-intelligent naïf and Maria’s gumption balanced by Clyde’s sarcasm.

In particular, a lot of effort is put into making Luke work as an ongoing, relatable character rather than the unearthly child of the pilot (in the same way that Susan’s ethereal weirdness lasts all of about 15 minutes of Doctor Who’s first episode). He’s still not quite got it: ‘Why is farting funny?’ he asks plaintively, and is fazed by the kind of unspoken rules the other kids live by, but he’s not incomprehensibly odd.

And then there’s Sarah Jane. Sladen’s clearly putting a lot of thought into her performance: the keen way she observes Maria’s interaction with Alan is so tangible it doesn’t really need the clarifying dialogue, ‘How do you get like that, you and Maria?’ At this point, the show is favouring the kids, with Sladen acting as a grownup ally, giving a steer to Maria, but largely following her own journalistic nose to investigate Coldfire.

This is ambitious, layered (Chrissie Jackson is another of RTD’s monster mothers; her flaw is utter self-orientation) and funny – like all the best CBBC programmes. Importantly, it doesn’t feel like a dumbed-down or patronising pastiche of Doctor Who, which would have been poison. The cliffhanger, of all the main characters menaced by unskinned Slitheen, is almost exactly the same as Aliens of London’s.

Next Time: Revenge of the Slitheen – Part Two