Now we all know that the two part Christmas special will be David Tennants last two episodes and his subsequent regeneration into Matt Smith's Eleventh doctor. But what else is in store?

From interviews and other news sites it has already been confirmed that Bernard Cribbings (wilf) will be back as the doctors companion for the episode. Also there is hints to more companions appearing and sharing the tardis. I doubt Rose will be back. Her story is finished. But I think it is possible for Mickey, Martha and Jack to be back. Maybe even Donna seing as her grandad is the companion.

However there has also been hints in a couple of episodes as well. Firstly we have "Your song must end soon" said by firstly the Ood anf secondly the woman in Planet of the Dead. This may be a referance to the doctors iminent regeneration which would seem very likely. But however I am not too sure. I think that it may be something else as it is not a mystery that the tenth doctor is too regenerate and so what would be the point of trying to build something up that everyone already knows. Secondly the Ood said this phrase before David Tennat knew he was going to leave didn't they? But I think that everyone will assume that it means the regeneration until BAM!!! it means something totally different.

Also at the end of the planet of the dead the woman gave us some more insights. We had the two phrases: "It is returning from the darkness" and "He will knock four times". Now my first thought about this is that it will ultimately be The Master. I beliee this because of a few things. Firstly throughout the whole 4th series we were repeatedly shown clips of the doctors hand and also Donna's ring. Now the hand played a vital part in the Master episodes as he used it to age the doctor. The ring however could be a referance to the masters ring which was picked up at the end of "Last of the timelords". It could even be the real thing. Also "He will knock four times" is a massive hint towards The Master as his beat in "The Sound of Drums" was four beats which was knocked on the table. These beats where also heard recently when the subwave entwork was opened in "The Stolen Earth" which I am sure used the same four beats. "It is returning through the darkness" could be a referral to Donna's memory. She has the Master's ring maybe and so as they try to stop the Master her memory returns through the darkness. This is why I think it may be the master.

Then again they may be referances for other things instead. it may be the Midnight Entity from season 4 episode 10. As this knocked four times when responding to the doctor and also Midnight links in from the Darkness. What else could it be? The darkness - Davros? The song - The Ood?

What are your thoughts, who will be there to aid the doctor? Who will he be fighting against? WHAT WILL HAPPEN? Please write your theories here and discuss what you think will happen!