The Final Game (unproduced TV story)

✅ The Final Game was an unproduced serial intended to end Season 11. It would have revealed that the Doctor and the Master were actually brothers which explains why the Master never sought to kill the Doctor. The story originally would have ended with the Master sacrificing his life for the Third Doctor's life and then the latter's regeneration. However, the episode was never produced due to Roger Delgado's death so TV: Planet of the Spiders replaced this story as Jon Pertwee's final episode.

Legacy
Several elements from this episode have been recycled throughout the years:
 * The Master caused the Fourth Doctor to regenerate in TV: Logopolis.
 * Their possible fraternal relationship was teased by the Master to the Fifth Doctor in TV: Planet of Fire but firmly dismissed by the Tenth Doctor after Martha Jones asked this question in TV: The Sound of Drums.
 * The Master sacrificing himself to save the Tenth Doctor from Rassilon in TV: The End of Time.