The fourth series begins with the nation’s favourite detective, the mercurial Sherlock Holmes, back once more on British soil, as Doctor Watson and his wife, Mary, prepare for their biggest ever challenge - becoming parents for the first time.
Sherlock waits to see where Moriarty will make his posthumous move. One mysterious case in particular baffles Scotland Yard - but Sherlock is more interested in a seemingly-trivial detail. Why is someone destroying images of the late Prime Minister Margaret Thatcher? Is there a madman on the loose? ...
Sherlock faces perhaps the most chilling enemy of his long career: the powerful and seemingly unassailable Culverton Smith - a man with a very dark secret indeed.
Holmes and Watson travel to Europe to escape Moriarty's vengeance.