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How did you come up with the concept for this new series?

"It was a story I'd had in mind for ages – I'm just glad the ³ÉÈËÂÛ̳ gave me a canvas big enough to tell the tale!

"But underneath the sci-fi and the aliens there's something very relevant to the world, I hope, the way we sit in the West and watch footage of atrocities in different countries and imagine it's all so far away, and so impossible here – which is a nice, comfy lie we tell ourselves. That was the heart of it.

"I wanted to tell a story in which civilisation snaps, in which we turn on ourselves, in which nothing is safe. Plenty of people live like that on this planet. In this story, it's Britain's turn!"

Torchwood – Children Of Earth feels very epic. Did you enjoy writing the one storyline over five episodes?

"I loved it, because it was a huge challenge. Lots of thrillers are written by just one writer, but we had three, across five episodes – which meant a lot of emailing and late-night phonecalls! But we really worked as a team, all locked in one room, to thrash out the storyline and create the characters, and that's my favourite way of working.

"We also had the producer and director inside the Writer's Room, right from the very start, which is a very unusual way of working in this country, but with huge results – it meant we were all focused, we all knew the tone and the ambition of the piece and we all aimed in the same direction."

The children of Earth are key to this storyline. Do you think using children is scarier than the actual alien?

"They just touch a nerve. A threat to our children gets a primal reaction out of all of us. But beyond that, I think we can be scared of our kids, too.That they can seem unknowable, unreachable – that's why a gang of young hoodies can seem more unnerving than an adult gang of thugs."

Do people need to know the back story to Torchwood to be able to enjoy this new series?

"No, not at all – there are fleeting references to the past but, from the moment it starts, we're telling a brand-new story. It's been deliberately written so that no-one will be lost – and, at the same time, the faithful viewer will discover so much more about the members of the Torchwood team! There are plenty of rewards for the long-term fan!"

The 456 is Torchwood's most fierce threat to date. How do you think the team deal with the situation?

"This whole story tears Torchwood down and then watches them rebuild, but always questioning them, asking what sort of heroes they are, how far will they go? And what's the difference between a freedom fighter and a terrorist?

"At the same time, we get to know Jack, Gwen and Ianto more intimately than we ever have before – exploring their families, their history, their hopes and loves and their failings, too.

"As the alien threat gets bigger, so Torchwood's humanity is exposed, and threatened, and celebrated, too. And their lives are on the line – none of them are safe!"

Tell us more about the 456.

"The size of this story, and the scale of it – spread across more than 40 years of history – means that we needed something bigger, a threat with real intelligence, a race with different protocols and standards.

"Some of my favourite material comes from episode three, where we have to see the government engage in genuine diplomatic relations with an alien species. You watch those scenes thinking: 'That's what it would really be like!'"

There are some fantastic guest stars in this series. What do you think they add to the series?

"Just pure class! It's a joy, an absolute joy, to work with actors of this calibre.Writing's easy, compared to the task of standing there, saying this stuff, in a whacking great close-up!

"But we've got great new talent like Cush Jumbo, as Lois – the innocent secretary who discovers state secrets on her computer – and wonderful stars such as Peter Capaldi, who makes his character of John Frobisher so detailed and so nuanced, and so heartbreaking in the end.

"Add to that Susan Brown, as Bridget Spears – keep an eye on her, she's a slow burner – and Nicholas Farrell as the most clever and manipulative Prime Minister you could imagine. And then there's Liz May Brice as a truly ruthless assassin!

"We've also got Paul Copley as Clem, a character holding so many secrets from the past – Paul's simply astonishing to work with.

"And then the greatest enigma of the whole series is Lucy Cohu, playing Alice, who's no less than Captain Jack's daughter… What a mix! It's the best cast I could have imagined!"

The relationship between Captain Jack and Ianto has developed. Was that something you'd be planning for a while?

"Not planning, as such, it just grew naturally out of the scripts and performances from John and Gareth. And it's such a rich area – the sheer will-they-or-won't-they tension of two men getting closer.

"But, again, you can come to Torchwood as a new viewer and follow their relationship from the start – you won't get lost! And it's honestly a pleasure to write for two such fine actors – they make the whole process a delight."

You recreate scenes of devastation in the middle of Cardiff Bay. Did you watch the filming of that?

"Oh, I stay away from filming, it terrifies me – far too much like hard work! But I did pop down to the Bay to see some of the devastation – and, as ever, a lot of it is plain old camera trickery! By choosing the right angles, and by adding beautiful FX shots, we were able to make the damage look a lot worse than it is. You can go down to Cardiff Bay today, and it isn't even scratched!"