 The final episode " His Last Vow " was first broadcast on 12 January 2014 , on BBC One , and written by Steven Moffat , directed by Nick Hurran and is based on Sir Arthur Conan Doyle 's " The Adventure of Charles Augustus Milverton " . This case leads Sherlock into conflict with Charles Augustus Magnussen ( Lars Mikkelsen ) , a " terrifying " villain who was introduced as an unnamed villain in episode one . Holmes and Watson try to get Magnussen arrested , but their attempt fails , and Holmes shoots Magnussen to stop him from blackmailing Mary Watson . Mycroft arranges that Sherlock will be exiled from the United Kingdom instead of being tried for murder . As Sherlock 's plane takes off , every video screen in London broadcasts the image of Moriarty , and Sherlock is recalled to deal with the crisis associated with Moriarty 's potential return . 
