The Doctor realises that they're breaking through from another dimension and tries to triangulate where from. Finally, Rose's mum gets to go on a ride in the TARDIS, much to the Doctor's horror.

Thankfully, they don't land somewhere on the other side of the universe. They land at one of the Torchwood bases in London.

The Doctor is immediately captured, his TARDIS impounded, and Jackie goes undercover as Rose, while Rose does some serious hunting about.

She discovers the Void sphere, a sphere that is built to travel in the Void between universes. Unfortunately her psychic paper is seen through and she's immediately captured - and covers her shock at seeing that Samuel, an assistant on the project, is really Mickey in disguise.

Torchwood are controlling the appearances of the Ghosts. Unfortunately the Cybermen have crossed over to this Earth and are systematically converting the Torchwood team into mindless Cyombies, with the plan of opening the Ghost system wider and permanently.

Eventually they do and all the ghosts in the world resolve themselves into Cybermen.

Unfortunately, at this point, the Void sphere starts to open and three Daleks emerge.

Well, 10 out of 10 for the surprises. Nice to see Mickey back, though that was expected. Great to see the Cybermen again but absolutely fantastic to see that the Daleks are back. Woohoo. I had thought that as Russell had dealt with the Daleks in Season 1, once and for all, it seemed, they wouldn't be returning anytime soon. But now, finally, after 40 years of waiting, we can all chorus as one - "The Daleks And The Cybermen Have A War!!!"

I'm not too sure about the beginning of the episode. If Rose is narrating it, how can she say that she died? Obviously she comes back to life somehow. Or maybe she's believed to be dead. Maybe she's trapped on another world at the end. Either way it leaves the series open for her return.

A number of news outlets briefly discussed the idea that the character of Adeola could become a companion of the Doctor. Unless time is reversed that's unlikely as she dies within the first few minutes of part 1.

8 out of 10. I hope the next episode is even better!

BBC blurb:
The human race rejoices as the ghosts of loved ones return home in the latest adventure from the nation's favourite time traveller. But as the Doctor, Rose and Jackie investigate the Torchwood Tower, the whole of modern-day Earth threatens to fall to an almighty invasion force.

David Tennant plays the Doctor, Billie Piper plays Rose, Camille Coduri plays Jackie Tyler and Tracy Ann Oberman plays Yvonne Hartman. 

The Cybermen and the Daleks meet for the first time in Doctor Who (Doctor Who - The Ultimate Adventure Stage Play is not counted!) so there is a great piece of cinematography where we see the Daleks activating their communications grid, cross to the Cybermen activating theirs, cross to a Dalek leaving the room and taking to the corridor to meet the new menace, cross to the Cybermen walking to the corner. Finally, after an agonising 30 seconds (plus about 40 years) the Daleks and Cybermen meet in the corridor. (Well, of course it's a corridor!) The Cybermen propose an alliance, the Daleks reject it and a fight ensues. Yes, Daleks are more superior than Cybermen, but we knew that already. When one Dalek can destroy a million people, a few upgraded people shouldn't be too much of a problem.

But what about the Genesis Ark?

Well, it's Time Lord technology. So, it's bigger on the inside. And, it's a prison ship containing millions of Daleks! The Dalek Invasion of Earth almost happened 150 years early.

Briefly:
Rose uses her knowledge of the Daleks and the time war to protect her and Mickey. The woman that was running Torchwood gets turned into a Cybermen. Jackie barely escapes it happening to her. Rose gloats to one of the Dalek secret order that she poured the time vortex into the Emporer Dalek and destroyed him. The Daleks need her alive because her time sensitivity means she can open the Genesis Ark and release a million daleks. The Doctor appears, explodes a few doors and helps the Cybermen attack the Daleks. Mickey falls against the Genesis Ark on the way out activating it. The Daleks open the roof of the building they're in and head skywards with it. Jackie is rescued by Pete Tyler while war against the Cybermen rages around them. Cars are exploding outside, Cybermen in people's homes, Cybermen marching around the Taj Mahal, Cybermen being exploded by bazookas on bridges. The Daleks release a million Daleks from the Genesis Ark and the Cybermen turn their attention on Torchwood tower.

The Doctor has been wearing 3D glasses to see the void stuff and can see that the Cybermen and the Daleks are steeped in it. All he needs to do is open the rift into the void, reverse the polarity, and send all the Cybermen and Daleks to hell. Unfortunately, because Pete, Mickey and the team from Pete's World are also steeped in void stuff, they'll need to go back to Earth2 before the Doctor opens the rift. And Rose has to go with them. Just as the Doctor is about to activate the rift, Rose returns and helps him, sucking millions of Cybermen and Daleks into the hole between worlds. But Rose's lever unlocks and she has to try and move it back to it's locked position otherwise some Daleks and Cybermen may be left. She locks it but the strength of the void is too much and she gets pulled, screaming towards the hole in reality...

Well, of course I'm not going to tell you what happens next!

In any case, this is a very teary episode so have your tissues ready. This is definitely a 10/10 episode. In fact it really is an 11/10 episode. 

Some trivia:
Earth 2 / Pete's World is about 3 years behind Rose's Earth! (Jackie is 40 in this story but on Pete's world, Jackie would be 43 if she was still alive.) 
If the Daleks shut down communications from their room so that Rose's mobile can't work, how is it that Jackie's still can. Just a localised communication black out then?
Why is the Doctor seen walking around the room on the video link but when we cut to the Doctor he's still standing next to Jackie talking with her?
Why didn't we see any Cybermen smashing out of people's homes when they're being sucked up by the void? Surely a few strong buildings could hold the Cybermen until the void closes? (There was a command by the cyberleader to converge on Torchwood so maybe all the Cybermen were out in the open. But what about some of the injured ones inside buildings?)
The Black Dalek activates a temporal shift, moving himself forward a few hours/days. Looks like Marsha Jones, Captain Jack, Torchwood and the Doctor will be fighting the Daleks again in Season 3!
This is not only the first episode in Doctor Who that features Daleks and Cybermen fighting, it is also the first episode to feature the Doctor crying, with tears running down his cheek.
At the end of the episode a woman appears in the TARDIS in a wedding dress. The Christmas Special is called The Runaway Bride and the announcer says that it will be screened in 170 days!

Here's the BBC blurb for Doctor Who - Doomsday
It's the end of an epic journey as two mighty armies wage war across the Earth, with the human race caught in the middle. But as an unstoppable terror emerges from beneath Torchwood, the Doctor faces an even greater dilemma – does saving the world mean the death of Rose Tyler?

David Tennant plays the Doctor and Billie Piper plays Rose.