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 actually kill the Doctor. The story 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 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 Logopolis and indirectly caused of the Tenth Doctor's regeneration in The End of Time. The possible fraternal relationship between the Master and the Doctor was teased in Planet of Fire and Smith and Jones but firmly dismissed in The Sound of Drums.