Fixed Point was always going to be a partial reconciliation for Chrístõ and his father. I had in mind from the start some sort of accident or terrorist act which would see Chrístõ having to let people die because their wounds were too severe. This, as his father rightly points out, is a desperate decision that is not too dissimilar to his own actions as an assassin.

We saw Chrístõ do this, of course, right back in the very first story, Elizabeth. It was stated then that he is the only person who could make that decision for others, because only he knew for certain that there was no hope. It is restated here, again. The idea that Chrístõ – and by implication – The Doctor – actually condones euthanasia is not one I want to promote, but the issue is one which is bound to come up for somebody with the sort of power over life and death that he has.

The Doctor has, actually, committed euthanasia, it should be pointed out. Though in very extreme circumstances. In Rise of the Cybermen, he ended the half-life of the cyber-converted Sally Phelan. Much further back, the Fifth Doctor ended the artificial life of the Kamelion. On occasion, he has been forced to do something that terrible. Chrístõ certainly isn’t going to make a habit of that. He knows it is immoral in most circumstances. He doesn’t condone it. Possibly, at some point, I could do with a storyline in which he opposes euthanasia in some way to balance things. I’m thinking about something on the lines of Logan’s Run, with a population culling programme. But that’s for another day.

The other subplot was Julia’s injury when the TARDIS was jolted by the explosion. Crush injuries are often fatal, of course. There is nothing trivial about that sort of thing. But this is science fiction and we have some tricks.

Romana, of course, is a healer. She removed the bullet from Chrístõ’s neck by telekinesis a few stories ago. But she isn’t able to help Julia without some remote assistance from Chrístõ and his father. I had the idea of Chrístõ talking her through a splenectomy, then realised that I didn’t know exactly how a splenectomy actually works. I do know that with internal bleeding the old, bad blood needs removing in order to prevent septicaemia. I could talk her through that. And at the same time it occurred to me that a blood transfusion was in order. The only person available to do that was Chrístõ’s comatose father, in the Zero Cabinet. He had plenty of blood to spare.

And of course, as I have shown on many previous occasions, Time Lord blood transfused into a Human, especially in large quantities, has the effect of giving the Human powers of tissue regeneration. The splenectomy wasn’t necessary. Julia recovered, and was able to talk to Chrístõ telepathically until he got through to the TARDIS underneath the rubble.

The TARDIS being in the rubble, actually protecting people, was an idea that came to me in the night while I was in the process of writing this story. I had a very vivid image of the survival space it might create. The idea seemed to work.

Chrístõ still had some issues with his father, but working together for the good of others was a start.