As the mystery unfolds, the episode reveals itself to be taking place within modern-day Sherlock’s "Mind Palace." While in a drug-induced state on a plane (immediately following the events of Season 3), Sherlock uses the Victorian case to help him solve the modern-day riddle of how James Moriarty could possibly have returned from the dead. Sherlock Holmes: Benedict Cumberbatch Dr. John Watson: Martin Freeman Mrs. Hudson: Una Stubbs Lestrade: Rupert Graves Mycroft Holmes: Mark Gatiss Mary Watson: Amanda Abbington James Moriarty: Andrew Scott

Often available for digital purchase or rental in most territories. Apple TV / iTunes: Available to buy or rent.

Occasionally available for free to viewers in the UK, depending on current licensing windows.