The Evil of the Daleks - S4-E9
Trivia: The Doctor only appears in pre-filmed insert scenes for the fourth episode, as Patrick Troughton was taking a holiday during production.
Trivia: In the animated version of this story, when we're at the police station, there are some wanted posters on the wall in the background. One of the wanted posters is for the original Master, Roger Delgado.
Other mistake: The Brigadier says UNIT HQ is 'a Top Secret establishment'. It is, in fact, so secret that it has a large sign outside informing the world not only of its function, but also the name of the commanding officer.
The Doctor: This is a situation that requires tact and finesse. Fortunately, I am blessed with both.
Question: What caused the first Doctor to regenerate? Was it old age?
Chosen answer: No, it was the result of the Cybermen's draining Earth of energy to their world, that also affected the Doctor, draining his energy as well, causing him to regenerate.
Also In the episode it's said that his body was wearing thin.
Dan23