I’m not one for much fan fiction but this is a pretty good Torchwood story. It’s titled “Countdown” and I’d estimate that it takes place shortly before Children of Earth (season 3).
It’s listed as Torchwood Virtual Series (Season) three, episode two so I guess I should look around for more.
If you read the story, but you’re not a huge Doctor Who or Torchwood fan, I’ll explain one detail that might not be clear in the story. UNIT stands for UNified Intelligence Taskforce (formerly United Nations Intelligence Taskforce) and is an Earth-based alien and paranormal task force; think of them as a more formal/military version of Torchwood. UNIT made a few appearances in the 2005+ Doctor Who series (recently and intergral to the plot of 2009′s Planet of the Dead special) and an occasional mention on Torchwood. UNIT is where Martha Jones has been working since she’s stopped traveling with The Doctor. I believe it was much more prominently featured in some of the older Doctor Who series (and might be make more appearances in the new 2010 Doctor Who).