Complex sentence: John Watson, an ex-army doctor injured in the war in Afghanistan is introduced to Sherlock Holmes as a possible flatmate by a mutual friend, sharing rooms at 221B Baker Street, owned by landlady Mrs. Hudson.
Split into simpler sentences: John Watson, an ex-army doctor injured in the war in Afghanistan, meets Sherlock Holmes through a mutual friend. They become flatmates, sharing rooms at 221B Baker Street owned by landlady Mrs. Hudson.