The First Doctor grew progressively weaker while battling the Cybermen during the events of The Tenth Planet and eventually collapsed, seemingly from old age. His body renewed itself and transformed into the Second Doctor.
Initially, the relationship between the Second Doctor and his predecessor was unclear. In his first story, the Second Doctor referred to his predecessor in the third person as if he were a completely different person.
During this incarnation, the Second Doctor confronted familiar foes such as the Daleks and the Cybermen, as well as new enemies such as the Great Intelligence and the Ice Warriors. It was during this time that he first met Alistair Gordon Lethbridge-Stewart, a military man who would later become the leader of the British contingent of UNIT, a military organisation tasked to investigate and defend the world from extraterrestrial threats.
The Second Doctor's time came to an end after he was put on trial by his own people, the Time Lords, for breaking their laws of non-interference. Despite the Doctor's argument that the Time Lords should use their great powers to help others, he was sentenced to exile on 20th century Earth, the Time Lords forcing his regeneration into the Third Doctor in the process.
The successful transformation into the Second Doctor, a figure who was the same essential character as the first but with a very different persona, was a turning point in the evolution of Doctor Who the series as well as the character of the Doctor, and was a critical ingredient in the longevity of the series.
He has been nicknamed the "Cosmic Hobo" as the impish Second Doctor appeared to be far more scruffy and child-like than his first incarnation. Mercurial, clever, and always a few steps ahead of his enemies, at times he could be a calculating schemer who would not only manipulate people for the greater good but act like a bumbling fool in order to have others underestimate his true abilities (The Tomb of the Cybermen, The Evil of the Daleks, and The Dominators). But despite the bluster and tendency to panic when events got out of control, the Second Doctor always acted heroically and morally in his desire to help the oppressed. More than any other perhaps, this incarnation of the Time Lord was a wolf in sheep's clothing.
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