I couldn't stop reading this - one of, if not the best, Doctor Who fanfiction I have ever read. I loved the way you got all the characters just about spot on, and some of it was very disturbing to read. And a great idea - usually most male pregnancy concepts are poorly done with little or no explanation to how it's done, with characters doing it just for the sake of it. And as other reviews have mentioned, an amazing account of the Doctor's "transformation" into the Master's willing slave.
I have nothing much to say except this is damn freaking good. I fact it's one of the best DW stories ever, and that's saying something. It hurts to read, but makes us come back for more.
Disturbingly realistic portrait of an individual being psychologically destroyed; realistic aftermath as well.
I read this series on your LJ rather than here, but since I don't have an LJ I thought it was better to comment here. I just wanted to say that I love this series. I've read a couple of very good AU to LotTL now, and this one is definitely on par with the great ones like MND, DW Season 4 and the Pandora Project. It's the darkest one I've read so far, but also the most memorable and gripping. After reading Zero Sum I actually lay awake for a long time pondering the story, which means it really got to me. I think you handled the Doctor's 'transformation' (to put it mildly) by the Master superbly. The way he's slowly, well, brainwashed into liking what the Master's doing to him, into trusting him so completely. And then Jack's story. Ugh, that really broke me. This series really is brilliantly written. I read on your LJ that you're going to write (at least?) one more part, so I'm very much looking forward to that. Oh, one more thing, I can't believe the frequency of the updates! Usually, people who write/update really fast either have very short chapters or the quality isn't very good. But you're able to produce a great story at a dazzling speed. You must be really inspired =)