Double Trouble was inspired by a storyline in Star Trek Voyager. Star Trek is not, by any stretch, a favourite programme of mine. It tends to be the only palatable thing on TV about eight o’clock in the evening when we have our evening meal and want something for background. On one particular evening, I caught part of a story in which the half-Klingon, half-Human crewmember, B'Elanna Torres, is split into two distinct people – her Human and Klingon halves. It occurred to me that, having pushed the idea of The Doctor being half-Human, half-Gallifreyan, I had the potential for a story. I considered it as either a Ten story or a New Lords of Time story featuring the Ninth Doctor, but since the finale of the 2008 series featured a Human version of the Tenth Doctor, it would probably seem too obvious. So I made it Chrístõ, the younger version of him, who became split into two beings.

Now, with the Star Trek version of this storyline, with a Klingon/Human combination, the contrast was quite obvious. The Human side was rather wet and feeble and was ultimately murdered. The Klingon side was aggressive and militant and despised the weak Human. My feeling about the treatment in that episode was that the Human B’Elanna was TOO feeble, really. The differences between a Human and a Gallifreyan are less pronounced. Chrístõ’s mother, as portrayed in my Marion and Kristoph series, was a woman who had a lot of courage and imagination. His Human side was never going to be a pathetic weakling. His Gallifreyan side was governed by a fair-minded and broad-thinking father who had travelled beyond the narrow confines of Gallifreyan society. Both sides tended to be more argumentative than Chrístõ usually was. His Human side was, naturally, governed by emotions. But his Gallifreyan side was, too, except that he covered it with Time Lord arrogance and a perception of superiority.

In fact, it wasn’t that his two sides were different, contrasting characters, but rather that they were alike except for the species denoted by their separated genes. They both wanted the same things. In particular, both wanted Julia to love them. And that was the thing that most of all made them realise they had a problem. Because by the time they finished arguing, Julia was starting to dislike them both.

This was a case of the sum of the parts not adding up to the whole. Either side of Chrístõ had shortcomings that the Human/Gallifreyan we know and love didn’t have. Julia needed her own Chrístõ back. His father would certainly want him back. And in any case, the future was bleak for both of them.

The solution was the chameleon arch. This device, first seen in the 2007 story, Human Nature, has proved very handy for a story idea already. I used it in the New Lords of Time stories, Son of The Master and Followers of The Master. This was a chance for Chrístõ to use it. Which would mean, of course, that the Tenth Doctor knew exactly what he was letting himself in for when he used it in Human Nature. He knew just how much it was going to hurt. But when did that ever stop him?

The trip to the ski resort between them discovering the solution and going through with the re-infusing of their two bodies was, to some extent, padding. But I also wanted to show a positive side to what had been a negative experience until then. The two versions of Chrístõ as friends, in a brotherly kind of bond, made it all the more poignant when they had to sacrifice their brief individuality for the ultimate peace of mind of both.