Third Doctor's regeneration

The Third Doctor underwent his regeneration into the Fourth Doctor as a result of radiation poisoning upon prolonged exposure to the Great One's highly unstable web of Metebelis crystals. (TV: Planet of the Spiders) He subsequently spent approximately a decade drifting through the Time Vortex, his body dying of the radiation (PROSE: Love and War), until the TARDIS brought him "home" to his laboratory at UNIT. This was the third regeneration of the 12-regeneration life cycle beginning with the First Doctor. (TV: The Time of the Doctor)
== History ==
=== A day to come ===
The Third Doctor briefly began to regenerate after battling a mind parasite known as the Nurazh. The Nurazh attempted to take control of his mind, but was overwhelmed by both the Third and Fourth Doctor's minds co-existing. The parasite parished, healing the Doctor and reverting the regeneration. Jo Grant witnessed this, and described to the Doctor his next incarnation. (PROSE: The Touch of the Nurazh)
=== Regeneration ===
The Doctor began the path to his regeneration when he elected to take heed of K'anpo's advice to return the "perfect" Metebelis crystal to the Great One, knowing it would lead to her destruction, as well as his own. Returning to Metebelis III, the Doctor was irradiated by the Great One's web of crystals as he gave the crystal to her. After she was destroyed, the Doctor limped back to the TARDIS. He "got lost in the Time Vortex", (TV: Planet of the Spiders) for ten years, the radiation slowly eating away at his body. While he did end up having some adventures even in his weakened state, the effects eventually became so severe that he could not even reach the TARDIS console, and was doomed to simply wait until the TARDIS landed of its own accord. (PROSE: Love and War, Ancient Whispers, The Loneliness of the Long-Distance Time Traveller) Eventually, the TARDIS brought him "home" to UNIT HQ three weeks after he left, and he collapsed outside. He said goodbye to Sarah and the Brigadier, before seemingly expiring. However, Cho-Je managed to give the Doctor "a little push" to begin the regeneration process, and the Doctor's face faded into his next incarnation. (TV: Planet of the Spiders) 
=== Interference ===
Due to interference caused by Faction Paradox, the circumstances of the Third Doctor's regeneration were altered, so that instead of regenerating on Metebelis III, he regenerated on Dust. This was because, after he transformed the planet into Foreman's World, he was shot in the chest by Magdelana Bishop due to the danger she felt that he brought to the village. When Sarah found him, the Doctor began regenerating, and as the process started, he knew something was wrong with the situation. Mother Mathara believed there would be no substantial difference in the timeline until the Eighth Doctor. (PROSE: Interference - Book Two) The Eighth Doctor eventually rectified this. (PROSE: The Ancestor Cell)
=== Legacy ===
Reunited with the Third Doctor in the Death Zone, Sarah Jane Smith reminded him of when he changed into what he knew as "teeth and curls", only for him to explain that it was yet to happen for him. (TV: The Five Doctors) 
== Behind the scenes ==
An alternative regeneration for the Third Doctor occurred in the stageplay Doctor Who and the Daleks in Seven Keys to Doomsday, which premiered after Planet of the Spiders and directly before TV: Robot. As both Jon Pertwee and Tom Baker were not available for the production, Trevor Martin was cast as a new incarnation of the Doctor. The play began with Martin wearing a wig and costume indicating that he was the Third Doctor, not unlike Seventh Doctor actor Sylvester McCoy's portrayal of the Sixth Doctor in Time and the Rani. He would then collapse on stage, and two "audience members" would rush from their sets to help him. An image on a rear-projected screen would then show Pertwee's face turning into Martin's, indicating that he was the Fourth Doctor. As the lights turned back on, Martin would be wearing a new costume with his natural hair. As he awoke, it was revealed that the audience members were his new companions, Jenny and Jimmy, who had been sitting in the audience as a trick of immersion.
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Category:Events involving the Third Doctor
Category:Events involving the Fourth Doctor