peeeeeeet's amazing 
tardis_bigbang fic, Two Smiths, which I had the great privilege of beta-ing a while back{*}.It really is astonishingly good. The jumping off point is a reconciliation of the inconsistencies between the NA and telly versions of Human Nature, but it's much more than that (and also far, far less fanwanky than I've made it sound). It explores all sorts of links between the new series and what's gone before, delving into the Time War, the relationship between the Doctor and the Master, and much more. It's a long story (almost 40k words, and if you're like me and normally recoil from that sort of thing with an instinctive "longfic is looooooooong", then believe me that you need to make an exception here) but it still feels jam packed with ideas;
peeeeeet is one of those people who do that thing I love of throwing away in an aside the sort of thing that lesser writers would use as an entire premise). It is, ultimately, a story about Doctor Who in a way that few are.You should really read it in order, but if you're wavering, check out what he does in chapter eight (it's a dream sequence, and that's about all the context you need) tying the Untempered Schism/sound of drums stuff into the wider Gallifreyan mythology. His implication about what the Doctor heard before he ran away is brilliant in both its audacity and its simplicity. As soon as I read it it went straight into my personal canon.
But just as importantly, the characterisation is excellent. Martha and Benny in particular are fantastically well-drawn, and go from being initially at odds with each other without it feeling at all contrived to bonding over the difficulties of companionhood. He nails Seven and Ten in both their Time Lord and human forms (and without spoiling it, there's an absolutely brilliant bit towards the end that would fall completely flat without him having them spot on) and the supporting cast are all well done too.
Go. Read it now. You won't regret it.
{*} Ahem. Huge apologies to
tanisafic fic for my being all FAIL on beta-ing her Mickey/Jake fic Momentum, which I do intend to read in full RSN (the bit I did read did a very good job with the ship; if you like them you should have a look at that too). I would like to blame RL, but I think we all know what I've been spending my time doing lately. Sorry. 