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. This story originally ends with Roger Delgado's incarnation of the Master sacrificing himself to save the Third Doctor's life and then the latter's regeneration. But the episode was never produced due to Delgado's death so TV: Planet of the Spiders replaced this story.

Legacy
Several elements from this episode have been recycled throughout the years:
 * The Master causing the Doctor to regenerate occurs in TV: Logopolis when Anthony Ainley's incarnation of the Master caused the Fourth Doctor to regenerate.
 * The Ainley Master showing horror at the thought of the Doctor deceased was implied in TV: The Five Doctors.
 * Their possible fraternal relationship was teased in TV: Planet of Fire by the Ainley Master to the Fifth Doctor but TV: The Sound of Drums dismissed this theory after the Tenth Doctor was asked this question by Martha Jones.
 * The original ending occurs in TV: The End of Time when John Simm's incarnation of the Master sacrifices himself to save the Tenth Doctor from Rassilon.