The Second Doctor is the name given to the second incarnation of the fictional character known as the Doctor seen on screen in the long-running BBC television science-fiction series Doctor Who. He was portrayed by character actor Patrick Troughton.
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.
With the arrival of a younger Doctor and changing tastes, the Second Doctor's tenure was characterized by a faster pace and a preference toward "monster of the week" style horror stories whilst the purely historical adventures that were a recurring feature of the Hartnell-era ceased with The Highlanders, the only Troughton-era entry in that genre. While Troughton's Doctor would still visit the Earth's past he would always encounter an alien such as the Daleks or the Great Intelligence. It was also during this era that Doctor Who began to come under fire for its purportedly violent and frightening content.
As with his predecessor, all the Second Doctor's original episodes were in black and white. Later guest appearances in The Three Doctors, The Five Doctors and The Two Doctors were in colour. However, Troughton's reign as the Doctor was more notable for what does not exist than for what does, as many of the episodes featuring the Second Doctor were junked by the BBC — a full list of incomplete Doctor Who serials shows how many of these episodes are missing from the BBC Archives.
Only one story in Troughton's first two seasons still exists in its entirety, ten stories only exist partially (most with one or two episodes out of 4 or 6), and four are lost in their entirety, including his first story, The Power of the Daleks; Jamie's first adventure, The Highlanders, The Macra Terror; and Fury From the Deep.
Some fans have also speculated that certain continuity elements within Doctor Who (particularly Troughton's appearance in The Two Doctors) suggest that the Time Lords used the Second Doctor as an agent after the events of The War Games, and that he did not in fact immediately regenerate and enter his exile on Earth. (See Season 6B for further details).
The Second Doctor would return to the series on three occasions: in 1973 for the 10th anniversary serial The Three Doctors (which also saw the return of William Hartnell as the First Doctor), in 1983 for the 20th anniversary special, The Five Doctors, and once more in 1985 in The Two Doctors. An official accounting of where these three adventures fit within the Second Doctor's chronology has yet to be offered, although there is longstanding fan speculation that the latter two stories might take place within the hypothetical Season 6B.