Besides their brief cameo appearances in "The Five Doctors", this is the first Doctor Who episode to feature both the Daleks and the Cybermen. They won't appear together on-screen until the next episode, however.

This episode and "Doomsday" were filmed directly after "Rise of the Cybermen" and "The Age of Steel."

Freema Agyeman (Adeola) will be later cast as Martha Jones, a companion of the Doctor, in season three.

Christopher Eccleston (The Ninth Doctor) appears in archive footage taken from "Rose".

Barbara Windsor appears briefly as her EastEnders character, Peggy Mitchell.

This episode marks the first appearance of a Black Dalek on the 2005 series.

Rose: (voiceover) Planet Earth. This is where I was born. And this is where I died. The first nineteen years of my life, nothing happened. Nothing at all, not ever. And then I met a man called 'The Doctor'. A man who could change his face. And he took me away from home in his magical machine. He showed me the whole of time and space. I thought it would never end.
The Doctor: How long are you going to stay with me?
Rose: Forever. (voiceover) That's what I thought. But then came the army of ghosts, then came Torchwood and the war. That's when it all ended. This is the story of how I died.

Jackie Tyler: You’re always doing this. Reducing it to science. Why can’t it be real? Just think of it, though. All the people we’ve lost. Our families coming back home. Don’t you think it’s beautiful?
The Doctor: I think it’s horrific.

Jackie Tyler: What happens when I’m gone?
Rose: Don’t talk like that!
Jackie Tyler: No, but really. When I’m dead and buried, you won’t have any reason to come back home. What happens then?
Rose: I don’t know.
Jackie Tyler: Do you think you’ll ever settle down?
Rose: The Doctor never will so I can’t. I’ll just keep on travelling.
Jackie Tyler: And you’ll keep on changing. And in forty years time, fifty, there’ll be this woman, this strange woman, walking through the marketplace on some planet a billion miles from Earth. She’s not Rose Tyler. Not any more. She’s not even human.

Rose: Doctor, they've got guns.
The Doctor: And I haven't. Which makes me the better person, don't you think? They can shoot me dead but the moral high ground is mine.

(passing Jackie off as Rose)
The Doctor: Here she is, Rose Tyler! She's not the best I've ever had... a bit too blonde. Not too steady on her pins, she's like that. Just last week she stared into the heart of the Time Vortex and aged 57 years... but she'll do!
Jackie Tyler: I'm 40!
The Doctor: Deluded... bless! I'll have to drag her in. Do you need anyone? She's very good at tea! Well, I say very good, I mean not bad... well, I say not bad...

Yvonne Hartman: The Torchwood Institute has a motto. "If it’s alien, it’s ours." Anything that comes from the sky, we strip it down and we use it, for the good of the British Empire.
Jackie Tyler: For the good of the what?
Yvonne Hartman: The British Empire.
Jackie Tyler: There isn’t a British Empire.
Yvonne Hartman: Not yet.

The Doctor: All those times I’ve been on Earth, I’ve never heard of you.
Yvonne Hartman: Well, of course not. You’re the enemy. You’re actually named in the Torchwood Foundation Charter of 1879 as an enemy of the crown.
The Doctor: 1879? That was called Torchwood. That house in Scotland.
Yvonne Hartman: That’s right. Where you encountered Queen Victoria and the werewolf.
Jackie Tyler: I think he makes half of it up.
Yvonne Hartman: Her Majesty created the Torchwood Institute with the express intention of keeping Britain great and fighting the alien horde.
The Doctor: If I’m the enemy, does that mean I’m a prisoner?
Yvonne Hartman: Oh yes. But we’ll make you perfectly comfortable.

The Doctor: This is a void ship.
Yvonne Hartman: And what is that?
The Doctor: Well, it’s impossible for starters. I always thought it was just a theory. It’s a vessel designed to exist outside time and space. Travelling through the void.
Dr Rajesh Singh: What’s the void?
The Doctor: The space between dimensions. There’s all sorts of realities around us, different dimensions, billions of parallel universes all stacked up against each other. The void is the space in between. Containing absolutely nothing. Imagine that. Nothing. No light, no dark, no up, no down. No life. No time. Without end. My people call it the void, the Eternals call it the Howling. But some people call it Hell.
Dr Rajesh Singh: But someone built the sphere. What for? Why go there?
The Doctor: To explore. To escape. You could sit inside that thing, an eternity could pass you by. The Big Bang, end of the universe, start of the next… wouldn’t even touch the sides. You’d exist outside the whole of creation.
Yvonne Hartman: You see? We were right. There is something inside it.
The Doctor: Oh yes.
Dr Rajesh Singh: So how do we get in there?
The Doctor: We don’t. We send that thing back into Hell.

The Doctor: So, you find the breach, probe it, this sphere comes through, six hundred feet above London, Bam! It leaves a hole in the fabric of reality. And that hole, you think, "Oh, shall we leave it alone, shall we back off, shall we play it safe?" Nah, you think, "Let's make it bigger!"

The Doctor: But when it made the whole, it cracked the world around it. The entire surface of this dimension splintered. And that's how the ghosts get through. That's how they get everywhere. They're bleeding through the fault lines. Walking from their world, across the void and into yours.

Yvonne Hartman: If that's Rose Tyler, then who's she?
Jackie Tyler: I'm her mother.
Yvonne Hartman: Oh, you travel with her mother.
He kidnapped me.
The Doctor: Please. When Torchwood comes around my complete history, don't tell people I travel through time and space with her mother.
Jackie Tyler: Charming.
The Doctor: I've got a reputation to uphold.

Yvonne Hartman: They're invading the whole planet.
The Doctor: It's not an invasion, it's too late for that. It's a victory.

Dalek: Location - Earth! Lifeforms detected! Exterminate! Exterminate! EXTERMINATE!

As Rose emerges from the TARDIS within Torchwood 1, a sarcophagus can be seen to her right. This references the original series episode "The Pyramids of Mars." UNIT HQ was built on the site of an old priory, where the Doctor battled Sutekh and his robot mummies in the 19th century.