Donna snuffs it. That's no big surprise - but there are a lot of rumours popping about - and some people at the BBC are also spreading them - that Donna isn't actually all she appears to be and has been infected by a spider that survived from the Xmas Special which introduced her.
Anyway what everyone seems to be agreeing on is that Donna dies - a noble sacrafice to save the universe - and the Doctor is so furious about this that he rewrites the rules - and time and space - and goes back in time so that Donna lives. He basically uses all his skills and powers as the last of the time lords to bend time to his will (apparently there's a thought that power corrupts might be used as an ongoing theme for a season with a darker doctor but I believe it's a tad early for that)
I'm sceptical, but I heard this one weeks ago and it seems to be gathering steam. I really hope the BBC is just getting good at false spoilers because the 'go back in time to save the day' ending was lame in the TV Movie and would be even worse here...
