Troughton was the first actor to put the idea of the Doctor's regeneration - a transformation which saw essentially the same character re-incarnated with a very different persona in another's body - into practice.
It is often considered as the turning point in the evolution of Doctor Who, becoming a critical element for the longevity of the show.
The Second Doctor's time came to end when he broke the Time Lord's laws of non-interference. He was sentenced to exile on 20th century Earth, forcing him to regenerate in the process.
Fun fact: Troughton commanded a gunboat in the second world war.
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