I think it was mentioned for the first time in the Tom Baker story The Deadly Assassin.
It was important to many storylines concerning the Master because he used up all his regenerations and resorted to stealing bodies from the 1970's to 1996.
If the Doctor can just regenerate over and over from now on without any explanation a lot of Classic stories can be thrown out the window including ...
The Deadly Assassin
The Keeper of Traken
Logopolis
Castrovalva
Time-Flight
Mawdryn Undead
The Five Doctors
Planet of Fire
Mark of the Rani
Trial of a Time Lord
Time and the Rani
Survival
Fox TV Movie
to name a few.
It was important to many storylines concerning the Master because he used up all his regenerations and resorted to stealing bodies from the 1970's to 1996.
If the Doctor can just regenerate over and over from now on without any explanation a lot of Classic stories can be thrown out the window including ...
The Deadly Assassin
The Keeper of Traken
Logopolis
Castrovalva
Time-Flight
Mawdryn Undead
The Five Doctors
Planet of Fire
Mark of the Rani
Trial of a Time Lord
Time and the Rani
Survival
Fox TV Movie
to name a few.
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