Cardiff railway station, 10am. The cab driver isn’t sure where exactly we are going. He pulls to a halt at the end of the rank, and hails the cab opposite.

“I’ve got passengers for Doctor Who ,” he says, with an expansive gesture at us in the back. “Where do I turn off?” “For Doctor Who ?” the other cab-driver says. “For Doctor Who? ” There is a long pause, where a more overexcited cab passenger might begin to speculate as to whether Doctor Who is shot on Earth at all. Maybe it’s accessible only via a closely guarded magnetic anomaly in a disused bronze mine, guarded by the Sontarans.

“You go right at the BP petrol station, mate.” Doctor Who and its spin-offs — Torchwood, The Sarah Jane Mysteries, Doctor Who Confidential and Totally Doctor Who — occupy Cardiff in much the same manner that an army barracks occupies a small town. With a 200-strong crew, 180 special FX technicians, 200 prosthetics technicians, 2,000 extras and 200 guest stars, the population of the city is divided into civilians and noncivilians; Who and nonWho. The pivotal question in Cardiff nightlife is “You on Who , then?”

“Some of them act a bit cliquey, like they’ve seen attack ships on fire off the shoulder of Orion,” says a friend who lives in Cardiff, “when in actual fact they’ve just spent all day waving a foam-rubber leg around.” But as with the Army, this clannishness is understandable — Doctor Who is both a huge and a hugely secretive operation. Having made the decision to try to keep the plots a surprise — extremely rare in television, where tabloid prepublicity is key in getting ratings spikes — phenomenal amounts of thought and energy are put into keeping details from the public. On the way to Cardiff the show’s press officer, Lesley, has a wary weather-eye out for possible leaks.

“We can’t discuss the show on the train,” she says firmly, as soon as we sit down. “People have done it before and had passengers who have overheard ring the tabloids. Everyone knows what you’re talking about as soon as you say ‘the Doctor’, you see.” So an hour later, when I am standing in a dark, otherwise deserted warehouse with the Tardis looming over me like the monolith in 2001 , I feel genuine frissons of both privilege and slight fear. Privilege because I am in a place where thousands of the show’s fans would love to be. After all, a mere 20ft away there is a top-secret spaceship being referred to as “the James Bond set”, which will titillate the spod glands of any Western adult between the ages of 17 and 50.

And fear because the Tardis — despite sitting on top of a pallet — looks unexpectedly legendary. It has the aura of something that has bounced off comets, arced over nebulae and oscillated through the farthest reaches of space-time. Even though, when I knock on its door, it is clearly made of wood.

The Doctor Who warehouse is a surreal place. Despite our last sighting of the Cybermen during Series 2, when an army of millions tried to take over the Earth, there are in fact only ten Cybermen in existence. Well, four now, due to breakages. I can see three of their legs poking out of a large cardboard box at right angles. The Daleks meanwhile are, contrary to all celebrity lore, actually bigger than they seem on television.

Being quite common, my first instinct is to steal something cool. I cannot be alone in this impulse. These warehouses are, presumably, an open invitation to cast and crew to take “mementos”. Everyone wants a Cyberman codpiece on their mantelpiece, surely? “To be honest, no,” says our tour guide, Edward Russell, brand executive of Doctor Who . “It’s like a family. It wouldn’t be worth their while because if they were caught they’d never work again. Everyone on this show is very protective.” He makes it sound as if, in the event of any transgression of trust, a hit-squad of Daleks might be seen trundling into a local pub and emerging minutes later with smoking ray-guns.

Of course, anyone venturing into an operation this big and, indeed, a universe this vast, requires a charismatic galactic chaperone. And as we all know by now, the resurrection of Doctor Who is down to one man — the joyous, expansive and prodigiously talented Russell T. Davies, the man who traded all his success with Queer As Folk, Bob and Rose and Casanova to do what the BBC had thought impossible for 16 years, namely to regenerate the abandoned Who and turn it into the BBC’s flagship. It is he, above all others, who is responsible for the best programme in Britain in the 21st century being, against all the laws of probability, a children’s show, made on a minuscule budget, in Wales, by gays. But perhaps Davies’s most crucial decision was his choice for the Doctor. For although in the first series Christopher Eccleston’s leather-jacketed, slightly demented hard-nut Doctor was the right man to make a break from the show’s heritage of frock coats, frilly cuffs and hammery, it is in David Tennant, the tenth and current Doctor, that the show has found its most appealing emissary.

While Eccleston approached the role prosaically as a difficult job to be done well, Tennant has taken it on with, well, love. A fan since childhood, he has been voted “The Best Doctor Ever” in acknowledgement that his performances, above all others, have best embodied the show’s values: anarchy, vigour, moral rigour, silliness and a reverential awe at how big, scary, complex, beautiful and full of bipedal aliens made of foam rubber the Universe is.

Meeting him in the tearooms of the Landmark Hotel in Marylebone, it’s clear why Davies cast him in the role. He has a quick wit, excess energy and self-deprecates at every opportunity (“Look at my mobile! It’s really boring! It’s about as intergalactic as a brick!”). He is also, let’s be frank, the first hot Doctor. He is the primary timephwoard. He was voted “Hottest Man in the Universe” by The Pink Paper , and New Woman magazine placed him at No 13 in its poll of 10,000 women’s crushes — just below Brad Pitt.

Tennant, however, disputes this assignation. “Tom Baker!” he says, with a Bakerish roar. “Come on! He was a huge hit with the ladies.” He was more of a specialist taste, I offer, primly. Something that WHSmith would keep behind the counter and you’d have to ask for.

“I’m sure Peter Davidson was in polls at the time,” he continues, gallantly. Perhaps aware that he is seconds away from attempting to mount a defence of the sexual allure of Sylvester McCoy, Tennant changes the conversation with a confidence that just, to be honest, proves how hot he is.

“This is a terrible anecdote, so I must tell it,” he says, settling into a chair with a coffee. “Last year Billie [Piper] and I kept getting invited to guest at award ceremonies but we could never go — we were either filming in Cardiff or we would be presenting Best Wig or something, and what’s the point of that? But when the Brit Awards rolled around, we let it be known through our ‘people’ that we’d love to present a Brit for Best Drunkard or something. But, pleasingly for the laws of hubris, they said ‘No, we’ll be fine, thank you’. They turned down the Doctor and Rose! Famous across the Universe!” Tennant does a self-deprecating boggle.

Talking to him is a mildly surreal experience. On the one hand, it’s the Doctor! You’re talking to the Doctor! On the other hand, he is as obsessive and passionate about the show as any fan. This is a man who can talk about the gravitic anomalyser without a protective layer of irony.

Dismissing the possibility that, paradoxically, becoming the Doctor could ultimately ruin the show for him — “I know what you mean, because all the surprises are gone, but I’d have gone mad if I’d turned it down and watched someone else do it” — Tennant instead spends the next hour discussing the show with all the enthusiasm and mild geekery of a fan, albeit a particularly privileged one. Discussing certain titillating morsels that Russell T. Davies has thrown into previous episodes, then not returned to — such as the intriguing news that the Doctor has, at some point, been a father — Tennant yelps and says “I know! I’ll be reading these things going ‘When are you coming back to that?’ Often he does. But sometimes,” he says, leaning forward, “he just drops them in for wickedness. There’s something he’s done in the next series, and I said ‘What’s that all about?’ and he replied ‘Oh, I’ve just put it in because it’s funny’. The internet forums will go into meltdown.” He beams.

“But you know, he knows what he wants as a fan. You want to be discussing it all the next week. You want to float different theories on what will happen next. That’s part of the pleasure.”

He comes across like a steam enthusiast who has taken over an old railway line. Every detail of the show thrills him, even the clothes. Indeed, perhaps the most surprising moment is when he explains how the image of his Doctor was, unguessably, based on the saviour of our fat schoolchildren, Jamie Oliver. “I’d always wanted a long coat because you’ve kind of got to. You’ve got to swish. Then Billie was on Parkie the same week as Jamie Oliver, who was looking rather cool in a funky suit with trainers. And I rang Russell T. Davies and said ‘Are you watching this? Could we do this for the Doctor?’ They had wanted me to wear a stompy pair of posh boots, but the trainers were the one thing I did go to the wall on.” Tennant bashes his hand down on the table, then laughs before adding: “I have to say, I do regret it when I’m doing a night shoot in a quarry of stinking mud and they’re putting plastic bags on my feet.”

The big news for the forthcoming series, of course, is that Billie Piper, who played the Doctor’s assistant, Rose, has left. As well as being phenomenally popular in the role — she was credited with bringing a young, female audience to a show that had previously lacked one — she and Tennant formed a famously matey duo. They always emanated the vibe of having spent their downtime in a Cardiff Nando’s, eating huge amounts of fried chicken with their hands and laughing with their mouths open. Discussing her departure, Tennant becomes quite tender.

“The last scene we shot was for [the episode called] The Satan Pit. Our last line was someone saying ‘Who are you two?’ and we (continued on page 6) reply ‘The stuff of legend’, then zap off in the Tardis. We just couldn’t get a take where we weren’t crying. If you look carefully you can still see us starting to go ‘Wah’.”

However, Tennant is stalwart in his enthusiasm for his new assistant, Freema Agyeman. “It’s a totally different energy — she comes from a totally different starting place. She’s very upfront about fancying [the Doctor], so he has to be very upfront about not being into it. It’s a completely new dynamic.” It’s Who 2.1, perhaps, I suggest. “Yes!” Tennant beams. “ Who 2.1!”

High on a deserted Welsh hillside, on the edge of a deserted slate quarry, Freema Agyeman, playing Martha Jones, films some of the last scenes in the new series. Something pivotal, which I’m not allowed to tell you, is happening to the Earth, in a time I’m not allowed to reveal, and Jones is watching the whole thing unfold from behind a rock. The rock, I can reveal, is grey and quite large. It is about the only thing within view that isn’t top secret.

Some 25 crew members are ranged around, tending to their roles. The sound team — always an outpost of licentiousness — have decorated their desk-on-wheels with a plastic flower nailed to a broom handle. One girl, possibly a runner, lies on her coat in the sun, texting sleepily.

Between takes, Freema bounds over for a chat. She is an immensely likeable, upbeat woman with a honking laugh, and it’s immediately obvious why Davies promoted her from a small role in one episode of the last series (an appearance now retrospectively explained as her having been Jones’s cousin) to one of the most high-profile roles in British television. Put simply, if you’re going to spend nine months of the year filming in a rainy pit, you want someone joyful and tough hanging out with you by the tea urn.

Agyeman is still in some measure of shock and denial over her new job. “I went to Selfridges last week to buy some beauty products, and asked the woman at the Elemis stall for some samples to takes home,” she says, casually twirling the Tardis key around her neck. “The next day my agent rang up and said they’d biked me this huge hamper of stuff — the entire range!” Before Agyeman landed the role of Martha, her acting career had been bumping along in such a half-hearted manner that she was working in her local Blockbuster, stamping out people’s copies of the Doctor Who box-set.

“Whenever I see clips now, I have a real insider’s view of the whole thing,” Agyeman says, swigging tea. “I look at what Billie was wearing in outdoor locations and check whether she could have worn thermals underneath. It’s often crucial.”

Breaking for lunch, the whole crew travels down the hill to the “base station” — a line of location buses and Portakabins. When Tennant turns up, dandy and wire-thin in his new electric-blue suit and precipitous quiff, the effect is roughly equivalent to the advent of the Fonz in Al’s Diner. He is clearly lord of this domain: he manages to hail, chat to and tease three crew members at once. By contrast, John Simm’s entrance is intense and low-key. As the pivotally evil Mr Saxon, Simm is in a black suit, wearing an ominous-looking ring and eschewing the buffet in favour of a quiet lunch in his trailer. “I can’t tell you anything,” he says, sighing. “I don’t think I’m even officially here, am I?” He shrugs.

Later on, in a waterfront bar back in Cardiff, Simm starts an admirably brisk line of whisky-ordering and explains exactly why he left a three-week-old baby to spend a month in Wales, on the side of a windy hill.

“It’s Doctor Who, innit?” he says, with admirable succinctness. “You’ve got to do it. And Christ, the energy they all put into it. Julia Gardner [producer] and Russell T. Davies were getting on midnight trains up to Manchester, to the set of Life on Mars, to ask me to do it.” The deciding vote, though, was cast by Simm’s five-year-old son, Ryan. “He’s Doctor Who mad. He’s got the lunch-box, the dolls, the screwdriver. As the dad of a small boy, you kind of have a moral duty to be a baddie on Doctor Who if you can, don’t you?”

Simm is keen to illustrate what he and Tennant have gone through to thrill this new generation of Who fans: just how far their dedication extends. “We were shooting one scene, just me and David, on top of this deserted mountain-top. We’re giving it our all when, from f**k knows where, you can hear the faint sound of an ice-cream van. David carried on so I thought, well, I’m not going to stop if you’re not going to stop. So we carried on right to the end, despite the fact that this must be the only ice-cream van in existence that does the theme tune to The Benny Hill Show — the least intergalactic sound imaginable!” He shakes his head. “We were, looking back, very professional that day.”

For Series 3 the BBC has taken the publicity for Doctor Who out of its own, often ramshackle, house and placed it in the hands of Taylor Herring, PRs to Robbie Williams, Big Brother and Al Gore. The new PRs, seemingly more aware of just how much interest there is in the show, have accordingly ramped up the screenings of the first two episodes. While screenings normally consist of a small room, 40 scruffy journalists and a table of coffee and buns, the Who screenings are treated like a movie premiere. Outside the Mayfair Hotel fans scream as a phalanx of paparazzi snap at the guests. While to judge from the celebrities present — Adam Woodyatt, Michelle Collins, Reggie Yates — it does look, by and large, as if someone took a van down to the BBC canteen and shouted “Anyone want to come and watch Doctor ’Oo ?”, there are sightings also of Jonathan Ross, Catherine Tate and Dawn French. Freema Agyeman is wearing a pair of £4,000 earrings, and both Tennant and Davies are resplendent in sharp suits, working the line like pros.

At the beginning of the screeningthere is, momentarily, no sound. The Tardis, iconic as ever, spins through electric-blue space-time to complete silence. Then the audience, as one, begin to sing the theme tune themselves: “Oooo WEEE oooooo/ OOOO ooo.” There is even an impressive counter-accompaniment of “De duddle le dum/ De duddle le dum.” It’s a moment of happy communal rejoicing.

Afterwards the consensus is that the potentially risky introduction of the new assistant, Martha Jones, has worked very well indeed, Tennant has ramped up his performance further, and the show seems set up to cruise into an even bigger Season 3. The very exciting kiss between Martha and the Doctor is being discussed at some length in the Ladies. Davies floats around, looking as joyous and serene as someone recently voted “Third Most Powerful Man in British Showbusiness” should, on pulling off another big success.

“The show is simply one of the best ideas ever, really, isn’t it?” he says, dragging on a ciggie and beaming. “So simple yet so complex. How can you not love a sexy anarchist roaming through time and space?” When asked if — given that Doctor Who has now, to all intents and purposes, overtaken EastEnders as the BBC’s flagship show — a larger budget would be more useful, he says a series of vaguely blustery and on-message things before roaring dramatically: “Yes! Yes! Yes, I want more money, goddammit!” And it’s hardly surprising that he does, considering that Who is still not being shot in HD — surely a foolish short-term economy, given the show’s inevitable longevity in repeats and DVD sales.

But all in all “I am a happy man,” Davies sighs, exhaling and staring across the room at the Doctor, his assistant and a circle of a dozen adults all squealing with excitement at being about to touch the Tardis. “A very happy man.” And he should, perhaps, feel a quiet satisfaction. After all, in a world where very little is a surprise and everything is viewed with cynicism, Doctor Who is a genuine rarity. It represents one of the few areas where adults become as unashamedly enthusiastic as children. It is where children first experience the thrills and fears of adults, and where we never know the exact ending in advance. With its ballsy women, bisexual captains, working-class loquaciousness, scientific passion and unremittingly pacifist dictum, it offers a release from the dispiritingly limited vision of most storytelling.

It is, despite being about a 900-year-old man with two hearts and a space-time taxi made of wood, still one of our very best projections of how to be human.

The new series of Doctor Who begins on BBC One at 7pm tomorrow

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