So, I wrote another 150 or so words of Master Plan at work today, and finally - finally - sorted out the couple of things I had left to work out. Most notably, I've finally worked out my take on the 'refusing to regenerate and getting Lucy to shoot him as his back-up plan' thing. This is something I have been trying to work out ever since I started RPing the Master (because I'm pretty sure it hadn't come up till then), which was the middle of December last year. So, eleven months, then. At least I've finally made it!
And so I have made some notes of things I want to include in this chapter so I now have a very, very loose plan for the shape of it. And I think there'll only need to be one more after that. I'm still working out exactly how I want it to end, because I'd always planned to work back to canon (i.e. Last of the Time Lords) but I need to decide how much of the stuff in that episode I'm going to recount. Don't want it to be a boring final chapter!
Anyway, I really feel that I'm getting there now. It's... a hell of a relief!