I was just thinking about this, most people are pretty sure that its The Master returning as John Simm was spotted filming but I saw this comentary on wikipedia from David Tennant & Julie Gardner:

David Tennant:
Really, from this moment on, the Doctor's card is marked. Because when we come back in "The Waters of Mars", it's all become a little bit darker"
Julie Gardner:
And as we know, David, he really does knock four times.
Tennant:
Yeah, absolutely, and if you think you've figured out what that means, you're wrong!
Gardner:
But when you do figure it out, it's a sad day.

The knocking 4 times is a reference to Last of the timelords but do you think The Master "knocking 4 times" could also be him killing ex companions or cast members?

or seperate times in history?

four subtle changes to history that change everything, history knocked off course....

Hmmm, I tend to think Tom is right about this one. Particularly with a new Doctor on the way we could be seeing all the companions except Jack dusted out to make a new start. But that's just me... who knows what could happen. It would certainly fit in with David's comment about it being "sad".

Im not so sure, a story like this would work if you took out the doctors 4 most important companions, the possibilities being Rose, Donna, Jack, Martha and Sarah Jane.

We know that 1.Jack Cannot be killed and 2.Killing Jack would also pretty much end any future series of Torchwood so he wouldnt be one of them.

The same goes for Sarah Jane, killing her would stop the Sarah Jane Chronicles outright as they all operate on the same timeline so to kill SJ or Cpt.J in Doctor who would look insignifiact and stupid if they then ignored this and carried on with each's respective shows.

So we only have 3 possible companions that bite the dust, unless they count one of the not so frequent tag alongs like Mickey or Rose Mum which also isnt that big of a loss in term of The Doctor, yeah he wouldn't be happy but it wouldn't have the effect of killing Rose.

The only way it could work if lets say the found a way to kill Jack and they did kill DN,RT,MJ and CJ or SJ the maybe The Doctor has to sacrifice himself to go back and change the timeline, he always says he cant go back on someones personal tmeline but maybe that is because its dangerous to him and on this occasion the sacrafice is nessicery to bring the 4-5 companions back.

Thats the only way I can see it happeing, I cant see them killing them all off for good but Ive been wrong before and I may live to eat my words, we'll just have to wait and see.

As you say, Jack and Sarah Jane have to survive or their spin-offs are kaput. By the way, it's the Sarah Jane Adventures, not Chronicles, I think you got confused with Terminator: Sarah Connor Chronicles!

Rose ain't gonna die, it would ruin the ending of Journey's End - she and clone Doctor were supposed to grow old together. Just trap her in the parallel universe permanently this time, and she's outta the picture anyway. As for Mickey and Martha, both would've joined Torchwood if Freema Agyeman hadn't been filming Law and Order and Noel Clarke hadn't been busy with his Adulthood movie. Since that could still happen, can't see either of them getting killed off either. Donna on the other hand, if she ever gets her memory back, the Doctor says she'll burn up - that could mean one of two things - death, or Time Lord regeneration. She is part-Time Lady now after all. If anyone dies, it'll be Donna.

Oh, another thing, if they use a changing history plotline, they'd be copying Primeval series 1's finale - so that's just not gonna happen either.

PMSL

Ive just noticed my mix up, what a dork

I think the Sarah Jane Chronicles is a far better title anyway!!

Sooo the Doc will turn up in sunglasses and ask Luke;

DOC10 "I am a friend of Sarah Jane. I was told she was here, could I see her please?"

LUKE "No, you can't see her, she's fixing K9".

DOC10 "Where is she?"

LUKE "Look, it may take a while. If you want to wait, you can sit over there"

DOC10 (looks around) "I'll be back..." (leaves)

Er... what's that stand for?

P*****g my self laughing

Im Imagining more The Master turning up in a ball of light completely naked, he walks into the intergalactic bar where The Doctor and Cpt.Jack are having a drink.

The Master " I need your clothes, your converse and your Tardis"

The Doctor " You didn't say please"

Cpt.Jack "Hi, Cpt.Jack Harkness, nice to meet you"

Looking down

"although id put that weapon away before I beg you to terminate me"

Doctor "Jaaack, stop it"

The Master gets bored and pulls a laser screwdrive from an undiscolsed location

Jack "Impressive"

Doctor "What?!"

Jack "I did that once"

Doctor "What?!"

Jack "Hey Doctor can we keep him"

Doctor "WHAT!?"

Doctor "Jack, quick come with me if you want to live"

Pointing the Laser screwdriver in the Doctors direction
Master "Hasta La Vista baby"

Looking confused
Doctor "Dont you mean Alonz-y"

I have too much imagination sometimes

Isn't that commentary directly from the Planet of the Dead Doctor Who Confidential?

To be honest, I'd heed Julie Gardiner's words. She says 'he really does knock four times.' Thus being actual knocks as opposed to metaphorical ones like.

Remember in Midnight a supposedly dark mind creature was trying to get into the marooned holiday shuttle.

Someone knocked..it knocked back.

Someone knocked twice...it knocked back twice.

The Doc knocked four times...it knocked back four times.

And they cannot kill Jack since he will become, millions and billions years after, the Face of Boe.