The Final Game (unproduced TV story)
From Tardis Wiki, the free Doctor Who reference
The Final Game was an unproduced serial due to end Season 11. It would reveal that the Doctor and the Master are actually two aspects of the same Time Lord and would end with the Master sacrificing his life for the Doctor. However, the episode was never produced due to Roger Delgado's tragic death and DW: Planet of the Spiders replaced this story.