Two years after his reported Reichenbach Fall demise, Sherlock, who has been cleared of all fraud charges against him, returns with Mycroft's help to a London under threat of terrorist attack.
I know, there are city names with articles (like "The Hague"), but I've never heard someone say "a London" till now.
Is "a London under threat of terrorist attack" used here as a description of some indefinite place in London?