Billie: Pop star. Teen wife. Time traveller

They've got Rose Tyler surrounded. Shop mannequins - as if by magic - have come to life and are closing in on the feisty 19-year-old. "Quick, in here!" A middle-aged man drags her out of harm's way. But before you know it, she's trapped again. This time, in a cavernous sewer space underneath the London Eye. This is the lair of a huge, glowing, pulsing blob of Play-Doh. It is the leader of the Autons, and it means to take over the planet, give evil powers to everything that's made of plastic, and kill all the humans. (Don't you hate it when that happens?) Now it's Tyler's leather-jacketed hero who needs rescuing. He's been rumbled for carrying a concealed sonic screwdriver. Two shop dummies have him in a firm grip.

They've got Rose Tyler surrounded. Shop mannequins - as if by magic - have come to life and are closing in on the feisty 19-year-old. "Quick, in here!" A middle-aged man drags her out of harm's way. But before you know it, she's trapped again. This time, in a cavernous sewer space underneath the London Eye. This is the lair of a huge, glowing, pulsing blob of Play-Doh. It is the leader of the Autons, and it means to take over the planet, give evil powers to everything that's made of plastic, and kill all the humans. (Don't you hate it when that happens?) Now it's Tyler's leather-jacketed hero who needs rescuing. He's been rumbled for carrying a concealed sonic screwdriver. Two shop dummies have him in a firm grip.

Thinks Tyler: "I've no job, no A-levels and no future. What I do have is a bronze medal in gymnastics." And with that she launches herself through the air, knocks out the dummies, rescues Doctor Who - for it is he - and saves the planet. Hurrah for Rose, the Doctor's intrepid new assistant! The Autons have been defeated.

The new series of Doctor Who offers many surprises. You'll believe a Dalek can fly, and that Christopher Eccleston - playing the Doc - can "do funny". And you'll believe Billie Piper can survive a brain-meltingly successful adolescent pop career, a much-discussed flame-out and a stint as the teenage trophy bride of an ageing media millionaire - and come out the other side, smiling. And sane. And sober. And rather impressive as feisty sidekick to a sparky but broody Doctor. Meet Billie Piper, Serious Actor. Who knew? Many people in the television industry, it seems. James Nesbitt, who starred with Piper in her breakthrough role, the BBC's 2003 update of The Canterbury Tales, can't rave about her enough. "Billie is an original, and has something that's a bit of a cliché - people call it star quality, but it's a very rare aura and presence as a person. That transfers to the screen." Eccleston - no lover of the celeb side of the biz - told me he clicked with her straight away.

"The camera loves Billie," says Pete Bowker, writer on The Canterbury Tales. "I've seen it before, but never witnessed it so dramatically." Brian Hill, who directed Piper in BBC2's 2004 care-home drama Bella and the Boys, says: "I needed someone who could play a 15-year-old and a 28-year-old. I saw 25, 30 actors for the role. She wasn't as experienced as some of the other people, but she had enthusiasm and an understanding - certainly of the 15-year-old. In the end, I thought she had something special. There's a steeliness there."

"She's got strength," echoes Russell T Davies. If the Doctor Who writer had experienced what Piper's gone through, "I'd be on cocaine in the gutter!" he hoots. "On our first day of filming, Cardiff city council had accidentally announced where we would be filming. We had 50 or 60 paparazzi waiting for Billie. I was anxious - I thought it might throw her. But she just ignored it and got on with her job. I was absolutely amazed by that."

But don't bother telling the starlet herself any of this. "It might be nice," Piper will say airily and non-committally, to be seen as a proper actor. "But I'm not really that bothered either way. Because as far as I'm concerned, I am a serious actress. I don't need any validation from other people to convince myself that that's what I am." She better hold tight, because if Davies - Britain's foremost contemporary dramatist, you might argue - has his way, she's about to have lift-off. "I think in five years' time," he coos, "we'll be sitting discussing her Hollywood career." Really? From Swindon, via Top of the Pops and Gallifrey, to La La Land? How on earth did that happen?

They've got Billie Piper surrounded. Tabloid journalists - as if teleported by BBC fun-bus from London - have pitched up at Cardiff's St David's Hotel and are pressing in on the 22-year-old. The occasion is the press launch for Doctor Who, which has been filming in the Welsh capital since last summer.

We enter the hotel through a mock-up of the Tardis, and enjoy our nibbles flanked by two Daleks. Billie Piper is encircled by microphone-thrusting hacks. They crowd round her, firing questions. She sits there at a small circular table, fidgeting with her meshy top, pulling the sleeves over her hands. In her breathy, mild Wiltshire accent, she answers everything.

Eventually, she begins to wilt. Piper finally finished filming the 13-episode series mere hours ago. It's been eight months of 11-day fortnights and 12-hour days. Last week she was on the front of the tabloids, dressed in her scanties, the pictures lifted from a "glamour" covershoot she undertook for a men's mag. "Dr Phwoar!" said the headlines. Doctor Who's new assistant, they roared, was - brace yourself - "dalek-table". Her builder dad in Swindon, unaware that the lads down the building sites were about to cop an eyeful of his eldest daughter, had texted her, "Cheers luv!". It's been a long old week. Can she go home now? Quick, in here! Finally a middle-aged man - a BBC press officer - drags her to safety. Billie Piper has survived to fight another day.

I had met Piper five days before the Cardiff beanfest. She was smiley and friendly but looked washed out. Pale and largely make-up free, her eyes were that of a junkie panda, and her complexion was spotty. Her hair was stringy, her roots badly needing a touch up. She wrapped herself in a shawl-cum-comfort blanket. "I'm a bit wiped out," she said. "I'm just surviving on coffee at the moment. Thank the god of coffee! It's my biggest vice right now." Did she have a post-Doctor Who holiday planned? "I'm kind of keeping my options open because, who knows, something great may present itself. Or nothing..." She couldn't wait to be done with filming and get back to London, not least because she was moving from Camden to a new place in Belsize Park. She wouldn't say if she was to be cohabiting with her boyfriend of four months, Amadu Sowe, a law student who used to work as a press officer at the record label to which she was signed.

Billie Piper has lived in the spotlight "since I left school". Is she aware that one-third of her 22-year-old life has been on the table, open to inspection? "I am aware of it. But also, because I started so young, and I had the pop career before I had my acting career, I'm not in it for fame or money. I'm just in it because I love it. That's a really reassuring thought. I just have to do what I want to do. I used to feel the pressure of, you know, maybe I should try this because the last time I did that, they criticised me for x, y and z. You just get to a point where you have to stop caring about that shit. Or just not reading it. I think when I met Chris..." A pause, then clarification, "...Evans, he helped me to deal with all that." Even before she married Evans - 16 years her senior - in Las Vegas in 2001 after a whirlwind courtship, Piper had packed a lot in. She grew up in Swindon, the eldest of four children. It wasn't a great place to grow up, but she had good times.

"I still get nostalgic when I hear a song that reminds me of sitting in a carpark eating a hamburger. But I just didn't want to do that, I didn't want to be there. I wanted to be in London, or in America. And I wanted to work. My dad always had such an amazing work ethic, it was something I wanted." At an age when most hormonal kids might want to shun their parents, adolescent Billie Piper "wanted to work like my dad". It was graft, not fame, she was after.

Aged 12, she landed a half-scholarship to the Sylvia Young Theatre School in London, beating 300 other hopefuls. "I am really excited as I've always wanted to be an actress," she told the local paper then. "This is the first big break for me." Boarding with relatives in south-west London and travelling by tube on her own was only the beginning of this provincial girl's preternatural self-confidence. When she was 14, she was cast in a promotional campaign for Smash Hits.

"We wanted to find someone who typified the reader," recalls Gavin Reeve, then-editor of the pop mag. "Bright, intelligent, sparky, a bit of attitude. The cool girl in the class. And loads of kids came in - wannabe acting girls with pushy mums, girls who obviously weren't going to be anything. Billie was by far the best." Piper was spotted in the TV ads and on the cover of industry magazine Music Week by Hugh Goldsmith of Innocent Records, a new label he had set up under the aegis of Virgin. He had previously worked with Take That, and would later have success with Blue and Atomic Kitten. Having passed his "audition" with a "protective" Sylvia Young, Piper, then 15, became Goldsmith's first Innocent signing. Launched in summer 1998 as just "Billie", she would sell one million copies of her first album. Her first two singles went to number one. Aged 15 and 16, she was - and remains - the youngest person to do that. Relatively speaking, she was more successful than Britney Spears.

Goldsmith: "I was instantly taken with her. She had that indefinable quality you look for in a star. A combination of a very strong look, very strong features. Almost like a beautiful Manga cartoon character. Huge smile, utterly charming, just a delightful girl. But interestingly, when I first met Sylvia, she said Billie's talents were, in reverse order, singing, dancing and acting first. Sylvia said she was a little sad that a singing opportunity had come along and Billie wasn't pursuing her acting career." Thirty intense months after the release of debut single "Because We Want To", and after her second album sold a disappointing (for the mainstream pop market) 250,000 copies, it was all over. Piper had three albums left on her contract but she abandoned her music career. She had travelled the world and played in front of President Clinton. But there were lurid tabloid tales aplenty. According to the red tops, she was a wild child who was out of control; she had a tumultuous relationship with Richie Neville of boy band 5ive; she was having abortions here and buying drugs there. Infamously, she was widely assumed to be the starlet Popbitch dubbed "Chazbaps", on account of her alleged willingness to allow men to snort cocaine from her breasts. (It should be noted that none of these allegations has ever been corroborated.) Goldsmith, a father of three, says he would never sign someone as young as Billie again - "too much responsibility".

Piper says that ultimately she coped with all the rumours, but she appreciates now how hard it must have been for her family. "There's pictures of me on the front of the Sport with my tits falling out at a concert. It must have been shit for my dad down at Jewson's getting his cement. I never thought about it at the time. Or my mum picking the kids up from school and nobody wanting to talk to her about her any more. It must have been terrible when people would say I was doing coke or having abortions every other week." Did she care when people were calling her Chazbaps? "No. I didn't read it... I mean, I knew that people were saying stuff ... But I really didn't care." She was similarly care-free, she says, when she was married to Evans and their life of moneyed, drunken, scruffy, globe-trotting millionaire leisure was being documented with forensic detail in the press. She was having too good a time. They were madly in love, both seeking ways out of professional and personal crises, and both making up for years swallowed by hard work. At the tail-end of her short, sharp, shockingly intense and successful pop career, she had been lost. Then she met Evans while doing a radio interview, and she was found.

Being with Evans was "so liberating" she gushes. "It was the most amazing experience. [For the first time] since I'd left school, I was doing exactly what I wanted to do, and on my own terms. That was very exciting. I had been groomed and made up and, you know, having to be awake and in tune every day, [so] I quite enjoyed... just being a free sprit and not really giving a fuck.

"And it's also something that I now use every day. The way I approach my life now is - just don't read anything. Don't worry about things. Even reviews and stuff - if you believe the good ones, you've got to believe the bad ones. It's just my survival kit." She is equally sanguine and forthright about their split last year. They were "inseparable for three or four years" but it had run its course. Evans seemed to blame her "back-on-track" career for the break-up. She says that when you're geographically in different places, it's difficult. But they're still best mates, she insists, always will be - a notion given credence by her game appearance as part of a little Evans on-camera prank during his hosting of The Brits last month.

Billie Piper said she has no idea what her public persona is. "And I don't really care." Certainly there's something of the "nation's sweetheart" about her. A bit Carry On, a bit Diana Dors, if you like. Brian Hill, who first had the idea of casting Piper alongside Eccleston - before they were teamed for Doctor Who - identifies her as the "girl next door, who's sweet and smiley but also has that sexiness. It's a good combination for a young actress. She doesn't have that threatening sexuality: women can like her as well. She's very user-friendly." Such was the success of The Canterbury Tales that the BBC are now giving Shakespeare the update treatment. Piper has just been cast in Much Ado About Nothing - she'll play Hero as a weather girl.

So where next for Billie? Is she the new Tamzin Outhwaite? Our next Catherine Zeta Jones? Or, with her film career so far amounting to useless Orlando Bloom boxing comedy The Calcium Kid, a low-budget British horror film called Spirit Trap, and a failed attempt to land the role Sienna Miller got in Alfie, is Billie Piper destined to be Barbara Windsor for the Noughties?

Outhwaite, says Pete Bowker. "I don't know that Hollywood quite understands how attractive Billie is and her look. And she doesn't strike me as the sort of person who would go to LA to hang around to get the odd part in the hope of getting a really good part in something like Traffic or Chicago. She strikes me as someone who needs to be working and needs to be doing interesting stuff."

Zeta Jones, declares an emphatic Russell T Davies. "She's phenomenal in Chicago. Billie could do that, if she wanted to, and will get parts like that: acting that good always finds work." A bit of all three, says James Nesbitt. "And many more besides. But I think she could give America a crack. I can see her having a career there. But ultimately she'll be Billie Piper."

Straight after our interview Piper was driving back to Cardiff for the final four days' filming on Doctor Who. She planned to stick Amy Winehouse on the in-car CD player: she likes the nu-jazz diva, likes her gobbiness. She knows her, in fact, because they were contemporaries at Sylvia Young's. Then she'll zoom up the M4 in her 1989 Porsche 911, mindful of the unmarked police cars that she thinks are an outrage (although she says she's only had one speeding ticket). She likes cars: when she and Chris Evans began dating in late 2000, he gave her a Ferrari 360 Modena (worth £110,000 then), covered in 90 red roses. She was 18, he 34. "It's like something out of a film," she said at the time. "I can't even drive."

She's no fool, Billie Piper. Despite her claims to the contrary, she knows how people see her. By her robust willingness to face up to any issue or rumour, by being no PR-protected china doll - and by being a good actress - she's claiming back her life and getting on with her work. She's managed to turn her music industry crisis into quality TV drama. Piper has learned how to perform, how to deal with the cut-throat entertainment business, how to protect herself, and how to move on and thrive. Which former Spice Girl or All Saint can we say that about? No, she says, she hadn't wanted to be a pop star. "I was just a huge fan of music, and that's how I like to keep it now. I don't ever want to go back to [music]. It actually gives me the fear just thinking about it." Now this young veteran watches "Fame Academy and all that stuff and I just think, 'Those poor kids...'".

She convincingly claims not to be stressed by how big a deal Doctor Who will be. She's done her bit, can't do any more. She won't be watching it on Saturday. "I'll do what I always do on transmission dates," she said brightly, "which is go to the pub and get lashed." Like the Autons and the Daleks, we underestimate Billie Piper at our peril.

'Doctor Who' begins on BBC1 this Saturday