The Inheritance

VALARIE: You mean out there right now there are families like mine, under threat from terrible things?

DOCTOR: Loads of them, unfortunately

VALARIE: And we can save them?

DOCTOR: We can try. It’s all we can do. And sometimes it’s enough.

VALARIE: Then yeah. I’m in.

DOCTOR: Well then. Welcome aboard, Valarie.

(Inside the TARDIS)

VALARIE: How does she work?

DOCTOR: Big bit fits inside the small bit. It’s like an optical illusion except with dimensions and not an illusion, so nothing like an optical illusion I suppose.