After the Doctor's health deteriorates while in the cold environment of the South Pole that the TARDIS has taken them to the Doctor, on returning to the TARDIS, collapses and then before his companions very eyes he regenerated into a much younger and dark haired man ("The Tenth Planet").

The Power of the Daleks

To his two companions this was not only a complete shock but also something that could not be possible unless this strange person is an impostor who has managed to enter the TARDIS. As Ben and Polly tried to challenge The Doctor they became more and more confused and sceptical, doubting whether the man before them was the genuine Doctor. 'Life depends on change and renewal' The Doctor comments (Although his habit of referring to The Doctor as a separate person at first did little to encourage the idea that he was still the same man). However, they are finally convinced that this 'stranger' is indeed The Doctor when a Dalek recognises him for who he is. This forces Ben to admit 'Well, if a Dalek takes you for the proper Doctor then I suppose I can' ("The Power of the Daleks"). Although some uncertainty remained regarding the new Doctor - with Ben once commenting that he wasn’t sure the new Doctor was ‘all there’ sometimes - he proved himself time and again to be just as capable as his previous self in fighting the evils of the universe.

Patrick Troughton played the part as a cross between Charlie Chaplin and the Pied Piper and was described by his first incarnation, when he meets him in a much later story, as a 'clown'. Significantly, he is the only Doctor whose clothing does not significantly differ from his previous incarnation, his favoured wardrobe being simply a larger, slightly baggier version of the First Doctor’s chosen garb, with the trousers now clownishly large and his cravat being replaced with a commonly poorly-tied bow tie. He still had a keen mind and sense of righting wrongs and he still managed to become entangled in the affairs of others, admitting to his companion Jamie that ‘I do tend to get involved with things’ (A marked contrast to his first self’s initial reluctance to get involved in the crises he found himself discovering as he travelled around in the TARDIS).

The Moonbase

However, his brilliant intellect and powers of observation would be hidden beneath his clown like façade and his tomfoolery made his enemies underestimate his abilities and motives; indeed, his clownish attitude was so perfect that even his future selves underestimated him, with the Third Doctor once commenting that the Second Doctor’s presence was more of a hindrance than a help ("The Three Doctors"), although the Seventh later described him as simply being more curious and prone to improvise rather than actually coming up with plans, encouraging his younger self to explore more detailed plans without actually condemning his traditional methods ("The Colony of Lies"). The Doctor’s childishness is well demonstrated when The Doctor ends up playing games with three playful Daleks who have been given 'The Human Factor'. However, when the Daleks’ true plans are revealed - to isolate the ‘Dalek Factor’ and spread it to humanity throughout history via the TARDIS - he tricks them into giving more Daleks the 'Human Factor' who then rebel against the Emperor Dalek ("The Evil of the Daleks").

 At moments of stress he had a habit of producing a recorder and playing a jaunty little tune - although only he thought that he was any good with the instrument, with others finding this habit to be particularly frustrating - or even occasionally dancing to a little jig. He was unable to hide his true feelings, clearly saddened at Victoria’s departure from the TARDIS, and was very modest much preferring to slip away quietly and unnoticed at the end of a story so as not to be caught up with the embarrassment of farewells or the answering of awkward questions. 'Why did you rush us back here for?' Zoe questions, on returning to the TARDIS after defeating the Ice Warriors attempt to invade Earth. For which The Doctor replies 'Well you know questions, explanations. It's very difficult. Come on.' ("The Seeds of Death"). In contrast to his desire to avoid the spotlight, he enjoyed chances to explore and have fun, being far more light-hearted than his past self, sometimes donning such bizarre disguises as secretary Lucy Buxom while in a role-playing game ("The Murder Game") or a gypsy-like musician in Atlantis ("The Underwater Menace") simply for the fun of it rather than any more practical purpose.

The Web of Fear

 A continued theme at this time from the First Doctor’s era was The Doctor’s inability to control the TARDIS, made evident on such occasions as The Doctor’s claim to be taking the ship to Mars ("The Underwater Menace") resulting in the TARDIS arriving on the moon ("The Moonbase"). On one occasion The Doctor claimed that this was the result of security measures installed by his people rather than his own inability to control the ship - the measures preventing a thief from taking the TARDIS wherever he wanted to go rather than preventing him from taking it in the first place - but his attempt to remove these protocols resulted in him ‘crash-landing’ the TARDIS in a pocket dimension containing a small American town ("Heart of TARDIS"), suggesting that his own lack of expertise played its part at this time. While he was able to control its flight on rare occasions, these were only under exceptional circumstances when following a powerful signal, such as when tracking a message sent from the space hotel Galaxian ("The Murder Game") or when he took a bomb directly to the heart of the resurrected Animus ("Twilight of the Gods"), both occasions requiring him to disable the TARDIS temporal circuits and more than a few of its traditional safeguards in order to be certain of tracking the signal.

 Unlike many other Doctors, who always retained at least an appearance of control even when faced with imminent death, the Second Doctor was the only Doctor known to actually show fear to his enemies, seemingly genuinely afraid when the Cybermen he encountered on Telos attempted to drag him back into their tomb as he fought to escape, and prone to the occasional moment of nervous chatter when faced with such ruthless foes as the Selachians ("The Murder Game"), or even a simple human mob (Albeit one under the control of an unknown intelligence) ("Heart of TARDIS"). Despite this, his resolve to fight the evil he encountered remained a dominant part of his character, stating more than once that evil 'must be fought' regardless of the cost to himself; when his traumatised companion Jamie believed that The Doctor was an android duplicate created to trick Jamie after the death of the ‘real’ Doctor, it was seeing The Doctor fighting the evil Captain Taylor that confirmed for him that The Doctor was real. He was also able to use his tomfoolery to bluff his way out of danger, such as in the "The Seeds of Death" when cornered by two Ice Warriors, who are about to shoot him, he announces in desperation 'Your leader will be angry if you kill me. I'm a genius!'

The Space Pirates

 As was the case with the First Doctor's era his exact origins were still never revealed, and only once does The Doctor mention his family and even then very vaguely. As he tells Victoria, who is still upset about the death of her father by the Daleks, that he can still remember his family 'I have to really want to, to bring them back in front of my eyes - the rest of the time they sleep in my mind and I forget. And so will you.' ("The Tomb of the Cybermen"). This scene with Victoria also demonstrates The Doctor's caring nature and his desire for the well being of his companions. This bond was generally reciprocated by his friends, Jamie in particular showing a great devotion to The Doctor as they travelled together; on one occasion when The Doctor was presumed dead, Jamie briefly became suicidally insane due to his guilt and grief over the loss of his mentor, but he recovered after he witnessed The Doctor alive again.

 However, the details of The Doctors true background were finally revealed during Patrick Troughton's last story "The War Games". After defeating an alien race, who have been using human soldiers from different war periods, The Doctor is forced to call upon the help of his own race 'The Time Lords' so as to return the soldiers back to their respective time zones. Fearing them immensely he manages to return to the TARDIS, along with companions Jamie and Zoe, but they find themselves trapped and so The Doctor ends up on trial for all the meddling he has done.

 Although The Doctor was initially sentenced to death - with Jamie and Zoe, being sent back to their own times with all memory of their travels with The Doctor erased - he was able to earn a reprieve by working for the Celestial Intervention Agency for a time, helping the Time Lords deal with situations where they couldn’t become involved due to their need to maintain a ‘hands-off’ policy in the affairs of the universe. On the first of these missions, The Doctor was granted access to new means of time travel, such as a Time Ring or a new, updated TARDIS, but on both occasions he stated his preference for his old ship and was eventually allowed to use it again, albeit with a recall circuit built in to recall the TARDIS to Gallifrey. After such missions as a confrontation with the mysterious Players ("World Game") - the stress of the adventure causing The Doctor’s hair to turn grey for the rest of his second life - and a near-transformation into an Androgum - which he only escaped thanks to some assistance from the Sixth Doctor - the Second Doctor eventually finished his time with the CIA, subsequently beginning his new sentence as he was forced to regenerate and exiled to Earth, his knowledge on how to operate the TARDIS erased.