Normally I’m not a big fan of AU crack, which is to say, fanfics where the author has thrown in something totally random (“what if Frodo and Sam became pirates?”) that really doesn’t relate to the original source.
There are exceptions.
It’s a fic for the new BBC series Sherlock, the one that updates the characters to the modern day. You don’t have to have seen the series, I think, to enjoy the story. But if you’ve read the Onyx Court books . . . yeah. Especially With Fate Conspire. It’s so much of what I think about London, in terms of its history and the relationship between a city and its people, with lots of little details that ring such familiar bells for me. gollumgollum pointed me at it, and I’m so glad she did.
Go. Read. Enjoy.