Looking at the poster for the film you can see Benedict Cumberbatch and make out at least three London landmarks: 1 The Gherkin, 2. St Paul's Cathedral and 3. the London Eye.
I assumed initially that they had just been thrown in there to tell you it is London, but using Google Earth I have found that they are near enough correct in scale relative to their real-world positions to each other.
And that vantage point in London is near-enough in line with the building used as the headquarters of the secret organisation dealing in extra terrestrial matters known as "Torchwood" in Doctor Who (2006 series).
These are the relevant buildings in Google Maps: A being Torchwood Tower, and the other three are the buildings seen in the poster
So, at this point my current theory is this:
'Secret organisation Torchwood captures and clones brilliant detective Sherlock Holmes, and due to the collapse of Torchwood in mysterious circumstances the clone is not activated until the 23rd century. Confused, angry, and possessed of a vast intellect, he tries to bring order and sense to this new world. Only one person can stop him. His name? Spock.'
Benedict Cumberbatch's character even says "I have returned."