“It’s the end, but the moment has been prepared for.”

That’s what the fourth Doctor tells his companions as he begins to regenerate at the end of 1981’s “Logopolis.”

And it’s that story that writer and producer Russell T. Davies looked to when it came to writing the final installments for David Tennant as the Doctor.

“It’s unusual. It’s got that kind of ‘Logopolis’ air to it, and I always liked that, because he knew from episode one that something doom-laden and sinister was on its way,” Davies says of the upcoming two part story, “The End of Time.” “This has got that funereal air of ‘Logopolis’ – but it’s not just funereal. You’re talking about five or six scenes.”

Interestingly, both stories will culminate the end of an era for the Doctor with the Time Lord facing his old nemesis, the Master.

Davies says that one of his regrets about the first time he brought the Doctor and the Master together was the lack of screen time together that Tennant and John Simms could share.

“It’s really, really hard to put the Doctor and the Master in a scene together, unless you start writing nonsense, like they say, ‘Ha ha, let’s team up to save the universe!’ And then the Master double-crosses him behind his back,” Davies explaines. “There are rubbish ways of bringing them together. But really, to put the Doctor in a room with a mass murderer is really difficult, because he’s not going to let him go. And he’s not going to forgive him, he’s not going to let him loose onto other people. So if you treat it as real then you really can’t have them together for long, because either one of them would do something about it. The Master would kill the Doctor or the Doctor would stop the Master.”

This time, Davies said he found the perfect way to bring the two together for the finale.

“They have much more screen time together this time. But there’s a limit to it. You can’t have great long scenes of them having a chat together, because they are absolute opposites. And I had no idea that John Simm is such a genre fan! I didn’t realise this the first time I worked with him, but you can sit and have a half-hour conversation with him about Superman and The X-Men. He loves all that sort of stuff,” he said.

Davies said he also wanted to challenge his leading man one final time in his performance as the Doctor.

Waving the sonic about and opening doors is not going to be a different day at work. Give him a challenge. Make him take a deep breath and think, ‘Wow, big day at work today…’ And then the camera crew does the same and the director does the same and everyone says ‘This is important.’,” Davies said.  “But you know, it’s the death episodes, they’ve got that built into them. It’s the weirdest game to play with the public: everyone knows it’s happening, everyone knows it’s on its way, no one quite knows how, no one knows how they’ll feel at the end of it… it’s brilliant, a really rare opportunity to do that. I hope we haven’t worn it thin! Sometimes you think there must be people thinking ‘Is he still there? Hasn’t he gone yet?’”

“The End of Time” airs in the UK on Christmas Day and in the U.S. on December 26.