Solace (I'm linking to the top of the post to give you a chance to check the lengthy warnings before you continue, but none of it's stuff that I consider sufficiently extreme to warn for here in the rec) is an awesome Handy/Rose fic of awesomeness. On one level, it's an astonishingly hot kinky porn fic of astonishing hot kinkiness, but it's much much more than that: it's a brilliant exploration of Handy, building on the ways in which he is like and unlike both the Doctor and Donna, while trying in an almost tragic way to be the Doctor, because that's what everyone, Handy included, expects of him.
It's written from an utterly believable Handy POV, and the sex elements (which are many, varied, and HOT AS HELL) are used to build a fantastic portrayal of the ways in which Handy's relationship with Rose over in Pete's World is based on being like the Doctor, but gradually evolving into its own thing, just as Handy becomes a separate individual. The whole thing has a wonderful sense of reality to it in the sense of being true to how people and relationships work in real life -- the ways the characters interact, even the fantastical elements related to Handy's Doctor-and-Donna origin (remember how I said it was hot as hell? the Donna/Lee flashback bits in particular are amazing), and especially the way things are not left too-neatly resolved at the end, are all utterly convincing. It's a brilliant piece of writing, and one of those rare fics that completely convinces me that it's written by someone whose handle on what's going on with the characters is not just as good as that of the canon creators, but better.