Eve

It's the apocalypse and 98% of humanity has been killed. Sherlock's older brother Mycroft has taken what remains of the British government and created a seed colony in an underground bunker outside London. On the outside there is chaos and gangs are 'collecting' stray loners and making slaves of them.

Sherlock is restless and goes scavenging in the ruins of London. He comes upon John Watson, who's been living on the streets by himself. He deduces immediately that John is an army doctor and tries to recruit John for the enclave. However, John suspects Sherlock is a slave collector...

Meanwhile, something else is going on. Mycroft's scouts are running into hostile forces. The collectors may be ganging up, with a new leader.

This story is written with a sort of gritty dark humor and has an actual plot. Sherlock is emminently Sherlock. :) Mycroft is..kinda scary. John is adorable.

As an aside, there are several stand-alone shorter Sherlock stories (not in this AU) by this author Archive of Our Own, and most of them are fabulous. Little bites of reading satisfaction. :)

However, while the ending to The Prize was a good ending, I would really like to see a sequel - I want more!!!