How did it all begin?

Julie Gardner [Doctor Who executive producer] asked if I'd do the Children in Need special. I thought it's be a lark, so I said yes. So you've got to write a seven-minute story, and there had been a very good one [for Children in Need] with David and Billie two years ago, which was the first meeting between his Doctor and Rose. That was a proper canonical part of the show, which I thought set the bar. So we had to do something that wasn't just a sketch.

Where did the idea come from?

I bumped into David at a concert and we talked about a way of getting Peter Davison back, because we are both big fans of Peter's Doctor. They hadn't had different Doctors meeting up on the new show; it hasn't been done for a long time [since the multiple Doctor stories of the classic series]. So if we could justify him being a bit older and pop him back in, that would be a laugh.

Steven Moffat, interviewed for the Radio Times, 10-16 November 2007