There's another, much darker and scarier version of London, and Sherlock finds his way there by accident. He needs to solve its terrifying mysteries to get home.

It can be the real thing, but I'd very much love if it was a horrible drug-induced hallucination (in that case I'd love John to take care of him and help him through it) and/or some sort of schizophrenic fit. If it's real though, I'd love John to be with Sherlock in that dark version of London (he'd be thrilled by the battleground like that, I'm sure, and would do his best to keep Sherlock's safe, while Sherlock investigates the whole nightmare).