HOWZAT

Several billion years in the future, she is all that remains of the purely human species, and she has definitely overdone the dieting, having become no more than a stretched film of skin with a face. Voiced by Zoë Wanamaker, she’s like Patsy in Ab Fab: bitchy and randy. “But she turns out to be murderous, and has a fantastic death,” says Motor Mouth. One other “signature dish” is the takeover of the bodies of the British cabinet by aliens.

This produces unfortunate amounts of gas, so the entire cabinet farts continuously.

What is interesting about all this is not just the campery of Davies’s imagination, but the sensitivity that makes it all work. He understands that the old formula has to change. Piper, the young assistant, for example, can’t just run around screaming: she has to have a background, and he has given her a family and a boyfriend. “She’s got a life — the old companions didn’t have a life,” he says. Equally, Davies understands that there are fundamental structural and stylistic elements that cannot be changed. The Doctor must be a little crazed, the assistant provides a curious, contemporary perspective, and the Daleks and the police box are just too much part of the brand to be discarded. The trick is to get the right balance of innovation and core values. “It’s a good idea, and good ideas never die. I love it when Disney does The Little Mermaid, and everybody says, ‘How dare they change the ending?’ But the Disney ending is better than the original. It’s like Beauty and the Beast, a great story that’s there to be told again, and they automatically become new stories. It’s a bizarre idea that they should just gather dust. Robin Hood, Tarzan, Sherlock Holmes — Doctor Who has the stature of those.”

This suggests that the shelf life of the Doctor Who idea is more or less infinite. But it requires intelligent protection. Even the best American sci-fi shows, such as Buffy and The X Files, went into decline too quickly. They, as Davies puts it, “vanished up their own mythologies”. Basically, the shows got so involved with their own setup that they became far too self-referential. “You can disappear up your own arse with your own continuity,” he says. “Those shows needed a good clean-out every three years. They get darker; they get wrapped up in their own very good stories.”

It is at moments such as these that you see the acute and intelligent critic lurking beneath the campery. But is he intelligent enough? Doctor Who is a huge gamble for the BBC. It will probably go out in its old slot, early on Saturday evening. This pitches it into the most competitive and difficult ratings moment of the week. At the moment, drama barely gets a look-in against Ant and Dec and all the other babble. The trick for the Doctor is to fight this with a show that grabs the children as well as their parents — just, in fact, what the old one did. I hope it works. Doctor Who is British sci-fi, full of native wit, charm and a strange sort of familiarity, a sense of belonging somewhere. “The thing about Star Trek,” says old Motor Mouth, suddenly wistful, “is that you can’t join in. It will never happen. But you can imagine walking home from school, turning a corner and seeing the Tardis. You could just walk in and join the Doctor. It could happen.”

Doctor Who starts on BBC1 at the end of March

Editor's choice

• The 100 Best Films of the Decade
• 70 facts you didn't know about Marvel
• The 50 Biggest Movies Of 2010