The latest Doctor Who special has been described as one of the darkest yet by Russell T Davis and David Tennant.
The Waters of Mars will be on your screens on 15 November and is the second to last adventure to feature David Tennant as the Doctor.
The story sees the Doctor travel to a human colony on Mars, 50 years in the future.
We see a new side to the Doctor while he battles his own demons and the creatures on Mars.
It is a frightening battle to watch as the Time Lord struggles with his role in the course of history and his ability to change it.
The Doctor goes it alone in this episode without the help of the a human companion, and he could really used a hand this time round.
David Tennant told Newsround that it was "probably" one of the scariest Doctor Who episodes.
"It tells a different sort of story. We expect the Doctor to take control earlier," he said.
This is one of the monsters the Doctor has to fight
And outgoing lead writer Russell T Davies said: "It's not so scary as intense."
When the people living on the mars colony are transformed into monsters it is up to the Doctor to save them and planet Earth, but can he do it, knowing what he already knows about the future?
So it's pretty dark and full of moments that will have you jumping behind the sofa but the man responsible for the creepy story line reckons that it is not too terrifying for younger fans.
In fact, Russell T Davis told Newsround that The Waters Of Mars will have kids as young as six glued to their screens and thinks it's exactly the kind of thing you guys should be watching.
And with The Waters of Mars out the way there are less than two hours of TV left with David Tennant in the Time Lord role, and he says he was pretty sad to film the last few scenes.
He says he does miss playing the role of the Doctor but was lucky enough to get a box of souvenirs when he finished the last show - and what do you give the Time Lord who has everything? His very own Sonic Screw driver of course!
David Tennant will say his last good bye to the Tardis during the Christmas special but will that fourth knock come and signal the end of the line for the Time Lord?