Sherlock Special: 10 Major Things We Learnt From The New Trailer

Sherlock's Mind Palace - the vast mental map he uses for storing information - is an increasingly oft-used device on the series. When Sherlock needs to deduce his way out of something we get treated to a trippy sequence set inside his head, usually festooned with fancy camera trickery.

And just because we're in Victorian times doesn't mean this sort of thing will be left behind, as this short moment from the trailer shows us. We see a glimpse of a crime happening in the street before the camera turns to reveal the Baker Street living room parked in the road. Presumably it's Sherlock visualising the scene. Or maybe 221B will be revealed as the TARDIS and so can materialise wherever it likes.

That would explain how they've travelled back in time...