Well, we first had the Tenth Doctor cross over into the Ninth Doctor’s world in the New Year episode. That went online a week after The Christmas Invasion, which introduced David Tennant’s Doctor on TV. And it was a bit hard on him. Both the Ninth Doctor and Rose were quite uncooperative and suspicious of him. The balance needed to be addressed to show the warm, sweet, nice guy that Ten actually was.

I also needed to launch a series of stories about The Tenth Doctor once his first TV series was over and the fans came looking for more. But the rumour mill was already turning on the idea that Rose wasn’t going to be his companion for much longer and in any case, running two versions of her relationship with The Doctor didn’t fit well.

So, THIS Tenth Doctor would be from further on in his personal timeline, and he would be trapped in Nine’s universe having crossed the boundary from his. Initially, I was going to make it possible for him to cross back and forwards, but I rewrote it slightly after the last episode of the 2006 series, because the clear message of Army of Ghosts and Doomsday was that crossing between alternative universes were dangerous. So the way back is closed. Nine’s universe has two Doctors in it, keeping out of each other’s way. The possibility is mooted of Ten having created a Duality. One version of him remained in his universe and continued to have a whole heap of his own troubles. The other came to this universe. This provided a relatively simple get out clause so that I could create new adventures for the Tenth Doctor without conflicting with the TV series. At the time of writing the story (January 2006), I had no idea what sort of new history The Doctor would be creating on TV. The rumour mills in the press indicated that Rose WOULD leave him one way or another. I had to be as vague as possible about that without giving too much away. As it was, the nature of her departure slotted in nicely. It was tragic enough for The Doctor without actually killing her off and without anything as boring as her going back to her normal life. It meant that the Tenth Doctor of these stories could still be feeling fragile about her, without giving away the reason for their parting. With a bit more information to hand, I am planning to explain in later Ten stories that the split occurred just after he met Donna Noble at Christmas 2006 and went off into space and time alone again. That means that he comes to this universe before meeting Martha Jones, and more to the point, Jean Redfern. Because I had a whole new love interest set up for him.

But meanwhile, Nine has a plan to make sure Ten isn’t lonely. Wyn’s character had proved popular, but really Nine and Rose needed quality time together. So she was written out of the Unfinished Business series here, to go with Ten and get into a whole new set of adventures with him.

http://www.pearsecom.co.uk/Ten/