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 or the latter Time Lord was the former Time Lord's darker personality. This story originally ends with the Master's first major incarnation sacrificing himself to save the Third Doctor's life and then the latter's regeneration. But the serial was never produced due to Roger Delgado's sudden death so TV: Planet of the Spiders replaced this story.

Legacy
Several elements from this episode have been recycled throughout the years:
 * Two elements were recycled in TV: Logopolis:
 * The Master being the Doctor's dark half was alluded towhen the Fourth Doctor implied this to Adric and Nyssa.
 * The Master causing the Doctor to regenerate occurs when the Master's second major incarnation caused the Fourth Doctor to regenerate.
 * The idea of a corporeal 'dark side' was used in TV: The Trial of a Time Lord, when the Master's second major incarnation (portrayed by Anthony Ainley) revealed that the Valeyard who had been prosecuting the Sixth Doctor was in fact a manifestation of the dark side of the Doctor's own personality, existing between his twelfth and final regenerations.
 * Their possible fraternal relationship was teased in TV: Planet of Fire by the Master's second major incarnation to the Fifth Doctor and Peri 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 the Master's sixth major incarnation (played by John Simm) sacrifices himself to save the Tenth Doctor from Rassilon, and the Doctor regenerates soon afterwards due to radiation poisoning.