I am addicted to this story! It's not Doctor/Donna per se (yes, in my brain it is, but I'm not sure that's the writer's intention. Or maybe it is...sneakily. I keep reading hoping it is.) Hell, ANYTHING can be Doctor/Donna, if you're in the right mood, am I right? Added plus, no Rose in sight.
Anywho, the story is the The Demon Saviour, by unslinky, found here.
I don't want to spoiler you, so take a gander at the first chapter and just SEE if you don't get hooked, too. Only problem is I've only found it on Teaspoon...Anybody know if it's posted somewhere else? I hate waiting for the admin approval before a chapter goes up. There the chapter is, just taunting you, and when you click it the abhorred access denied message comes up. Argh! Someone, enlighten me - is it somewhere else?
UPDATE as of June 2013! SPOILER SPOILER SPOILER --
BEHEMOTH STORY with over 200 chapters! Turned out to be NOT Ten/Donna (although there is ONE very good smut scene between them) but if you aren't into the pairings that happen, you may be a bit miffed at the amount of time you put into it. :) Still a very well-done story.