"Right. Um. About time I explained about this place, I think." The Doctor took another sip of his drink, shaking his head to the offer of another.
"We're in a sort of bubble universe. A place where different times and different realities collide. Or at least, the people from them collide. People get - dropped - here, seemingly at random, and then, well, they can't get out. Until one day, again apparently randomly, they suddenly find themselves back where they were, the exact moment they left, with no memory of this place at all until they return here."
He paused for breath, and looked anxiously at John to make sure he was following.