DOCTOR: But between you and me, I haven't got a plan. No idea. No way out.

ROSE: You'll think of something.

DOCTOR: The entire Earth's been sterilised. This, and other place like it, are all that's left of the human race. We might hold out for a while, but nothing can stop those creatures. They'll get through in the end. The walls aren't that old. And there's nothing I can do to stop them. There used to be laws stopping this kind of thing from happening. My people would have stopped this. But they're all gone. And now I'm going the same way.

ROSE: If I'd realised.

DOCTOR: Just tell me you're sorry.

ROSE: I am. I'm sorry.