In an exclusive interview, Billie Piper tells Newsround about her role as the companion in Doctor Who.
She plays Rose Tyler, a teenage shop assistant, who joins the mysterious time traveller on his adventures in time and space.
Tell us about your character.
Rose is a 19-year-old girl and she lives with her mum on the same estate she's lived on for 19 years.
She has a very normal job and she's looking for something to happen. She's very inquisitive and she's just looking for someone to challenge her ideas.
And sure enough this 900-year-old man rocks up in a blue box and wants to take her on some serious adventures!
There's this whole new world and he's going to broaden her horizons, and really test her and she's going to learn from him and she understands that.
And she just goes for it, and she's quite ruthless - she completely drops her life as she knows it for these crazy journeys in time and space.
What's the Doctor like?
The Doctor is like no-one she's ever met before. He can be quite cold at times and he can be quite stand-offish and alien, but he challenges her - no-one has ever challenged her so much.
And he's sees something fantastic in Rose and that she has huge potential, to be someone really, really great. And he's funny and crazy and eccentric and wild.
She's never met somebody as odd as this before, but also as brilliant as this before.
And she certainly never met anyone that celebrates the human race as much as he does.
What's the story about in episode one?
Episode one is an introduction to Rose and the Doctor. It shows you a bit of her life before it all kicks off.
They meet in her shop and then they just become the best of friends. Towards the end she has to choose which life she wants to live.
What monsters do we see in episode one?
We see the shop window dummies - that's what I call them. And that's your introduction to the spooky side and the sci-fi. But it gets worse and it gets a lot more interesting and a lot more life-threatening.
What attracted you to this part?
I heard they were bringing Doctor Who back and obviously I understood the show and how iconic it was and its history, although I was never really a fan as a child.
I mean we had it on in the house but I can't remember really sitting down with my family to watch it.
But I just loved what was on the paper. I loved the script, and obviously the idea of working alongside Christopher Eccleston and putting Russell T Davies' words into action was, for me, the greatest appeal.
How did you get the part?
I auditioned for the role of Rose and then I had a recall and in my recall I had to read opposite Chris which was absolutely terrifying! I mean here's someone who I admired and enjoyed and it was just such a daunting experience.
But he was so lovely and so calm and collected and we just got on famously.
What is the scariest moment for Rose in this series?
I think the scariest moment for Rose is when she meets her father. Her mum is a single mum and her father died when she was very young.
It's a concept I think we all toy with when we encounter death - what would it be like is he was still around? Or what would he look like now?
What about the scariest monsters?
There's an episode with very spooky children - that's all I can really say, but for me just hearing a child's voice echoing around the room, that was the icing on the cake. It was absolutely terrifying, it was haunting.
Do you think Doctor Who will scare young children?
I think the show has a perfect balance of sci-fi and is also very domestic and kids will be able to relate to the human situations.
But the slightly more sci-fi elements of the show can be quite scary at times. I don't know how scared they will be, I suppose we'll have to wait and see, but it is spooky.
And it's heavy, it's hard work, it's not light-hearted, it is a romp essentially, it's a load of fun but it's also quite mentally stimulating and I think kids will really enjoy that.
If you could travel in time, where would you go and why?
I'd like to see what my 30s look like, not too far - we're talking about eight years down the line. I'd like to see what's going on in my life, that's quite interesting to me.
Doctor Who is on BBC One at 7pm on Saturdays.