DOCTOR: I could take you forward -

SAM: No. We've figured it out. I'm going to be staying here. Sarah's got a couple of big projects lined up in the next year or so. She's going to need help. And there's a spare room here, so I won't get in the way of... you know. Her private life.

SARAH: I'm hoping there'll be a Nobel Prize in this somewhere.

SAM: Either that or we'll end up bringing down Western civilisation.

"Everybody's dead," said the Doctor. "That's one of the problems with time travel. Everybody's always dead, and everybody's always alive. It's all a question of where you're standing."