Russell T Davies on The Stolen Earth - Radio Times, June 2008

Benjamin Cook talks to Russell T Davies about bringing back Davros and building up to the fourth season's big finale, a two-parter starting with The Stolen Earth.

"The most important thing about the show is the Doctor himself, and the Daleks raise the stakes for him," says Russell T Davies, Doctor Who's showrunner. "He witnessed the Daleks' creation, fought them in ten different bodies. He saw them murder the Time Lords. He lost Rose to a parallel universe because of them. So when the Daleks appear, it gets personal."

This year's two-part series finale boasts a menagerie of different Daleks. "We called it the Dalek Freak Show! We've got the Supreme Dalek, a mighty red metal beast, and he's truly impressive...although everything's in danger of being upstaged by Dalek Caan. Our voice man, Nicholas Briggs, gives an amazing performance. It's the creepiest Dalek yet!"

Also returning - for the first time in 20 years - is Davros, creator of the Dalek race. "I hope older viewers get a shiver of recognition," says Davies, "but I really want kids to love this bizarre half-man, half-Dalek." How come he's waited until series four to resurrect such an iconic adversary? "It's given a chance for the myth to grow. The legend becomes a bit of a game, between us and the viewers, between parents and kids. I'd like to think that mums and dads have been stoking the rumours! The longer you leave it, the more currency it gains. But we've waited long enough. Besides, he's my favourite monster. So I waited to give myself a treat!"

Julian Bleach is the fourth actor to don Davros's latex mask. "Julian has it all - the voice, presence, intelligence, even the physicality. Davros might only have one arm, but Julian makes it incredibly expressive and muscular. And wait till you see him tip Davros into insanity - it's terrifying!"

Is Bleach the best Davros yet, then? "Now, don't be cheeky. I love all those Davroses! But Julian is a rightful successor to the throne, bringing elegance to the part, even a cruel playfulness. Davros is contrary to everything the series stands for: life, survival, optimism, hope and fun. But Davros's world is so twisted and dark, it's like staring into the pit. Yes, he's clever, he's a genius, but all his brilliance has gone into creating the darkest force in the universe."

And as well as Davros, look out for various plot strands drawing together. "There have been fleeting mentions of missing planets," says Davies, "and you're about to discover their fate. Donna keeps mentioning the fact that, back on Earth, the bees are disappearing - that's going to become important. And way back, in our very first episode in 2005, the Ninth Doctor invoked the might of the Shadow Proclamation - well, here it comes!"

And there have been "mysterious hints" about Donna's future, which "will reach a terrible climax. That said, this episode is still a rip-roaring adventure that you can follow without having noticed any of this stuff."

Last month, the BBC announced that this series of Doctor Who (and next year's specials) will be the last in charge for Davies, who was awarded an OBE earlier this month. A tough decision? "Do you know what? It was easy," he says. "I decided way back in 2006, so it was more like keeping a promise - with the satisfaction of a job well done. I'm proud of every single episode we've made, and that's a good time to leave. I know my own mind, and I know for a fact I won't regret it.

"When you choose to be a writer, you choose to be freelance. I've never stayed in a job this long, and I've only stayed because I've enjoyed it so much. I don't even know where I'm going. That's so exciting." He breaks off. "Oh God, I sound like the Doctor!"