Inferno

One of the striking things about Inferno - which, at only the fourth story, concludes this first season of the new Doctor - is that unlike the other three, it doesn't obviously rip off (or rather, homage) anything from Nigel Kneale's Quatermass serials. It's also remarkable for being quite unlike the Doctor Who of previous seasons, and what was to come. It's a fascinating story that rewards re-viewing.

Okay, so there is an initial family resemblance to the TV work of Nigel Kneale, and particularly his famous creation, the rather Doctorish Professor Quatermass. We're in another sinister industrial unit, and we're digging into the surface of the Earth. It's almost a reimagining of The Quatermass Experiment, heading in the opposite direction - trespassing beyond human experience, uncovering a violent potential in ourselves.

And at this point, whilst it would require a heavy rewrite, this could have been made with Patrick Troughton's Doctor. The green ooze that possesses and transforms men into monsters is not a million miles away from the stinky seaweed that did similar stuff in Fury from the Deep. It would actually feel quite derivative if this was Patrick Troughton poking about in machinery and arguing with the base's hotheaded boss.

But Inferno takes things in an entirely new direction for Doctor Who. It is closest in genre to the dreamlike tales of the Celestial Toyroom or the Land of Storybook Characters that the 1960s toyed with so memorably. Here is the programme's cosy set-up (only four stories in, albeit over twenty actual episodes) inverted. The Inferno disaster itself becomes almost secondary to the actual story. You could easily substitute the events of Spearhead or Silurians here. It's a nightmare world.

You couldn't tell this story with the Second Doctor, Jamie and Zoe - or rather, you could, just about (evil Jamie, evil Zoe) but it would have about as much substance as a toot on a recorder. Liz and the Brigadier belong to 'our' world, and that they are different tells us, immediately, how different this other world is and how high the stakes. It's more fun - more exciting - and there is more potential in our understanding of those characters, even if we fans can get more from our speculations than the writer gives us in dialogue, the emphasis being more on drama than psychology.

It's the new Doctor's first Tardis trip, and it takes us somewhere more alien than Dulkis, to his biggest challenge in a long time - alone and friendless, with no handy gizmos, no means of escape or even shelter (how different it would be if the whole Tardis had transported him), trying to win the trust of a paranoid people. The most shocking thing is that he succeeds, but also fails, horribly - escaping with his life but no-one else's. I'm watching The Mind of Evil this week, and there's a subtle allusion to Inferno, the Doctor describing a fiery apocalypse as a memory that still haunts him.

Jon Pertwee's Doctor is less mercurial than his predecessor. Having to defend himself in that kangaroo court seems to have hardened his character - he's loud, flamboyant, outspoken, angry. He no longer sneaks about in the background or covers genius with bluster. But in this season, we've seen him new facets of his character that are compelling. Put through several ordeals, he remains eminently himself, a loud voice in favour of peace. This story alone stretches Pertwee's performance: he's a stellar leading man, and he makes the Doctor a vital, convincing character.

It's strange - looking back from the vantage point of Season 8 - seeing what a strong character Season 7, necessary for a 'dark mirror' story like this. It's telling stories its previous era never could, or never tried to. It's ignoring all the easy choices - Daleks, Cybermen, Yeti. Inferno demonstrates how many elements it can draw into one story - and make work: the Doctor's role at UNIT but also his desperate longing for escape, his sense of our world as home, the scale of the threats he can predict, his limited powers, and what our world is like without him.

And a last word should go to Caroline John, who leaves us on a laugh - a laugh at the Brigadier (after telling him, 'I don't quite care for your tone!') and the Doctor (who she has watched take his opportunity and fly away from our world in his mysterious Tardis, or did he die?). She's an amazing performer and I've loved her portrayal of Liz. One more season of this team would have been wonderful - going into the strange relationships of the characters, in these lovely long, slow, wintry stories of folk horror and our fragile planet. And more Liz Shaw, and Liz in space, and Liz saying goodbye. It's a credit to these four marvellous stories that they leave us wanting more.