Donna: So come on, then. Robot Santas. What are they for?
The Doctor: Oh, your basic robo-scavenger. The Father Christmas stuff's just a disguise. They're trying to blend in. I met them last Christmas.
Donna: Why, what happened then?
The Doctor: Great big spaceship, hovering over London? You didn't notice?
Donna: I had a bit of a hangover.