It's official; you're BRILLIANT at this.
I particularly liked the explanation you came up with for Erika being Reinette; plausible, interesting, and yet another example of how the Elders can be a bunch of backstabbing bastards even WITHOUT the factor of being hybrids.
Sometimes, some people are just born jerks, I guess.
Still, I cna't deny it added a lot to the crisis; the thought of the likes of Marcus getting his hands on the TARDIS is an absolute NIGHTMARE...
Of course, there was another factor that I found particulary good; the Doctor's newly-restarted relationship with Reinette. For most of the other Doctors, I would have said that he was too human, but I've always found the Tenth Doctor to be VERY human- more so than any of his predecessors- so that little detail is easily excused as Erika teasingly reminds him of their forgotten past and Reinette resolves to explore everything they can be to each other once again.
Talking of the Doctor's reactions, I particularly approved of his reaction to Corvinus' reluctance to kill Marcus and William; I can DEFINITELY see the Doctor losing his temper at the man's inability just to kill his SONS when the Doctor had to blow up his entire PLANET...
The confrontation with Marcus was another well-written scene; I just wish the Doctor had managed to use some of his alien martial arts on the bastard, as well as doing the whole trick of managing to SHOOT a superhumanly-fast vamp/lycan hybrid.
Regardless of that, however, the idea of HOW the Doctor stopped Marcus was excellent; the man aspired to be a God, but his detachment from the world meant that he was vulnerable to the oh-so-human condition of emotion...
Well, at least the good guys won in the end.
Keep up the good work!
...Any chance of more stories showing the Doctor and Reinette?