 The first episode , " A Study in Pink " , loosely based upon the first Sherlock Holmes novel A Study in Scarlet , was written by Moffat and directed by Paul McGuigan . The story depicts the introduction of Sherlock to John , and them entering a <unk> at Baker Street in London , and then their investigation into a series of deaths , initially believed to be suicides . Mycroft Holmes , Sherlock 's older brother , played by Mark Gatiss also appears for the first time . The episode was first broadcast simultaneously on BBC One and BBC HD on 25 July 2010 . 
