The script contains the words 'Emily', 'pipes', 'carrots', 'legend', 'carrot' and 'concerto' and the phrase 'the map of the Water Snake Wormholes'
RTD in SFX says, "There's a very strong link between the last three specials. It's not a three-parter, but there's a strong connection."
Set Reports
A lot of this episode was filmed inside - so most of what I say is just guess work (and I guess a lot, mostly because its a fun word to type).
Anyway... The main setting is some sort of giant greenhouse on Mars:




And judging by the trailer, this bit of the building is important. Either because its got a rocket attached (which means escape!) or because its got some solution to the problem, or maybe both!
So why is there a big greenhouse on Mars? Well, what with a later scene having this sign in the background:
Methinks that the purpose of the greenhouse is to generate more oxygen for a nearby starved planet... or something like that. Sound plausible?
Scene ??The TARDIS was seen on location (when they were filming in a quarry) but it wasn't used in any of the scenes. So I'm guessing we might see Doctor exiting out of it at the beginning?

Scene ??
And inside this greenhouse thingy the Doctor meets this lot, who I guess are running the greenhouse with Adelaide as the Captain.
"State your name, rank, and intention"

Scene ??
But then, as always, some immensely creepy and horrible thing happens and members of the crew start being possessed...
And of course, if you go wandering off on you're own with only a torch, you're bound to get in some serious trouble.
Scene ??
But whatever the danger, the Doctor has these comforting words:

"Certain moments in time... are fixed. Anything else is in flux and anything can happen, but those certain moments, they have to stand... This base on Mars. What happens here must always happen."

"This thing, whatever it is, doesn't just hide in water. It creates water. And if you take that thing back to Earth... One drop...."
Scene ??
And then I guess there'll be lost of action and running away from the scary possessed people while they try to kill/possess the others:

"Don't drink the water. Don't even touch it."
Scene??
I really don't know where this fits in... Until now, we've mostly seen two evil water guys running about being evil and wet. But as Adelaide says:
"Any one of us could be infected."
And from the looks of it she's inside, where everyone else is! But then again, all the flashing red lights generally never mean anything good, so perhaps she's about to be deadified anyway...
Scene ??
But of course, the Doctor needs to go be heroic and save the day, even if Adelaide doesn't think its the best idea.
"They said I was going to die. They said 'he will knock four times' and I think I know what that means and it doesn't mean right here right now! 'Cause I don't hear anyone knocking, do you?"

*KNOCK x4*
Scene ??
But out he goes anyway... And what happens to the crew that are left?
*THUMP THUMP*
"They're on the roof..."
Dangerous stuff!
Scene ??
This either happens much earlier on, or right near the end... or maybe neither, I have no idea.
"All systems online, 100%, not a single delay."
"Don't you worry Captain, we are gonna fly!"
In the trailer, the pushing of the button is linked with the big explosion at the end, but I always get the impression that the explosion is more to do with the Doctor.
Scene ??
And where's the Doctor gone while the others are trapped? Methinks it was outside.
And why's he gone outside? Methinks it was to blow stuff up.
Or to run away from stuff thats blowing up.
In the words of Scooty: An explosion the size of which I have never seen before. It formed a giant mushroom cloud and left a huge black smoke ring in the sky.
Who or what causes the explosion? I've got no idea. But the Doctor manages to survive. Here we see him running away from the explosion (in this case, green screen) and being blown forward (through the use of a trampoline)

Scene ??
The Doctor then wakes up amongst a load of rubble. And slowly walks forward. Quite probably very bewildered and whatnot.
Something else survives the explosion to - what seems to be a friendly robot can just about be seen through the flames

And of course there are other survivors as well. I'm not sure how they survived but they did. Maybe they were just in a different area of the greenhouse...
Scene ??
And then from there... again I've got no idea where all these scenes fit into the episode, but this one does feel quite like an ending one, with a nice cliff hanger.
The Doctor, Adelaide & two other crew people emerge from the TARDIS in the middle of a snowy street.
The robot is also there (it has the word "GADGET" written on its front) and it seems friendly as it trundels forward to talk to the Doctor.
Ahremsee managed to beautifully capture the scene in this video.
Although you can't really hear whats being said, we know that halfway through (when the Doctor looks up) he says 'its snowing! I love snow.'
And according to Victoria Feller, the line immediately after this (said by the male companion) is something along the lines of: 'its not snow...' as he holds out his hand. So more ash from some destroyed ship/planet/greenhouse thing?
After a while two of the crew leave and we're left with the Doctor and Adelaide together. I don't know what goes on between them (probably a "goodbye/no you can't travel with me" moment) but then the Doctor points his sonic at a house that she goes over to.
And then it gets REALLY interesting.
As the Doctor heads back to the TARDIS alsone, it seems like he hears something. He looks to his left, shocked and aghast, slumping against the TARDIS and then falls to his knees. After this it seems he then gets up, rushes into the TARDIS and slams the door.

A horrified Doctor
Picture courtesy of The Gather
And a video of the Doctor falling to his knees
According to Lucky Lady, they apparently filmed two versions of this scene. One where the above events happened, and the other where the Doctor simply enters the TARDIS.
The reason he gets so freaked out? Oh, let the speculation commence... the robot? A returning villian? Or... and I'm not positive of it, but this could very well be the reason the Doctor looks to freaked out:

Oh yes, Ood Sigma is back!
So goodness knows what cliffhanger this is leading off to! According to Scooty, he doesn't interact wit the characters, but just seems to appear at the end of the scene...
And according to RTD the Doctor is reaching the end of his life and may have some nightmare type things because of that. So is Ood Sigma just an illusion?