Lestrade notices a stray cat hanging around his and Mycroft's house and starts to do little things to make her life easier. An occasional tin of cat food, a box with a blanket over it behind the hedge to give it a bit of shelter, that kind of thing. And Mycroft does not approve, not by any stretch. Apparently Mycroft is not a cat person, or even a pet person. Cats are "pestilence incarnate," a "demonspawn" Mycroft not-so-affectionately renames Leviathan. But Gregory is persistent. It's one of the cleverest, funniest bits of domesticity I think I've had the pleasure to read.
This is slash of the Mystrade variety, and Greg and Mycroft bicker like an old married couple. That means there are some references to sexytiems, but of a thoroughly PG variety. Really, it's just good solid humor, and I highly recommend it.
Check it out "There Are Always Exceptions," by EventHorizon.