Human Nature (Paul Cornell)

 Having been restored to himself, The Doctor gave the pod to the Aubertides, the first person to activate it quickly realising that they had been tricked. Expecting to filter out the mind of a Time Lord while absorbing the biodata in the pod, the Aubertide was unprepared for the sudden influx of human nature, resulting in him being transformed into Smith, who subsequently sacrificed himself by setting the Aubertides’ equipment to self-destruct. After Smith had a brief conversation with The Doctor on an astral plane while he died - during which The Doctor assured Smith that he had done a fairly good job as The Doctor -, the Eternal Death came to take Smith, satisfying a prior deal The Doctor had made with Death to save his companion Ace ("Love and War"), Smith serving as one of The Doctor’s lives for Death to claim in Ace’s place. Saying goodbye to Joan on Smith’s behalf, The Doctor assured her that Smith had genuinely cared for her even if he couldn’t feel that way himself, Joan giving him her cat Wolsey as a reminder of the time she had spent with Smith ("Human Nature").

John Smith (Human Nature/The Family of Blood)

  Despite the complications that had resulted from his time as Smith, The Doctor was forced to assume the John Smith identity once again in his tenth incarnation, albeit for drastically different reasons. While the Seventh Doctor became Smith for a holiday, the Tenth Doctor assumed the identity in an attempt to escape The Family of Blood, a race of incorporeal aliens who wanted his DNA to extend their life span, attempting to give the Family the chance to abandon their search for him and live out their natural lifespan in peace as he feared the more ruthless methods he would need to use to stop them himself. Using the Chameleon Arch - a device capable of rewriting Time Lord DNA and turning them into humans, presumably developed to protect Time Lords during the Time War -, The Doctor transformed himself into a new ‘incarnation’ of John Smith, once again hiding in 1913 in a similar manner to how he had hidden originally, his companion Martha forced to work as his maid (Why The Doctor chose such a hiding-place given Martha’s racial background was never specified, but it may be that he simply lacked the time to find a better time to hide because he was worried about the possibility of the Family finding the TARDIS if he remained in flight for too long).

It is interesting to note that, in many ways, the Tenth Doctor’s take on John Smith was drastically different from the Seventh’s, due to the fact that he developed some of humanity’s less admirable traits where the Seventh’s Smith had retained most of his Doctor-inspired instincts. While the Seventh’s John Smith retained The Doctor’s core values about issues such as violence while being drastically different from him in terms of personality in areas such as his ability to come up with elaborate plans or his capacity to relate emotionally with others, the Tenth’s Smith seemed to be far more a product of his adopted time while being otherwise fairly similar to the Tenth Doctor in terms of his basic personality, such as his knack for on-the-spot improvisation (Albeit limited due to his knowledge being restricted to what he knew of Earth in 1913) throwing a cricket ball on a complex trajectory to stop a pram being crushed. The change in personality was most particularly shown in Smith’s ‘new’ relationship with The Doctor’s companion Martha Jones, forced to assume a disguised identity as Smith’s maid to keep an eye on him; while The Doctor had occasionally been unintentionally dismissive of Martha’s feelings for him while still treating her as a close friend, Smith was far cooler towards Martha due to her skin colour and her status as a ‘servant’, to the point where he briefly laughed with Joan Redfern - the school nurse and his current love interest - about Martha’s seemingly mad ‘lies’ after he fired her when she tried to tell him the truth about who he was.

Although both the Tenth Doctor and Smith were more willing to express their emotions than the Seventh Doctor had been, the Tenth’s Smith still lacked The Doctor’s ability to be ruthless when the situation required it, appearing genuinely terrified of the situation he found himself in when the Family managed to track him down even when he took up guns to defend himself (Another action distancing him from the Seventh’s Smith, who had never taken up weapons himself). With the Family having taken the bodies of local residents to use as hosts, and the watch containing The Doctor’s biodata having been unintentionally stolen by Tim Latimer, one of Smith’s students, Smith was left increasingly confused and terrified at what was happening to him, retreating to a nearby house with Joan and Martha even as the Family revealed that they had managed to capture the TARDIS. When Tim appeared to return the watch to Smith, he explained that his contact with the watch had allowed him to experience part of The Doctor’s life, his admiration for the Time Lord, coupled with Martha’s pleas, forced Smith to accept the necessity of turning back into The Doctor, although the watch gave him and Joan a chance to see the life they would have lived if he had been real.

Having turned back into The Doctor, The Doctor posed as Smith long enough to hand the watch over to the Family while setting the ship to self-destruct by seemingly randomly flailing about and pressing certain buttons. With the crisis over, The Doctor offered to give Joan the chance to travel with him, but, in a mirror to what had taken place with the Seventh Doctor and his Joan, Joan turned The Doctor down because she couldn’t love him now (Although the evidently traumatic nature of the transformation raises the possibility that The Doctor was being slightly influenced by the remnants of Smith when he asked Joan to come with him) ("Human Nature/The Family of Blood"). The Doctor briefly visited Joan’s granddaughter Verity Newman when he was about to regenerate ("The End of Time"), learning that she had published her grandmother’s journal of her time with Smith as a book, The Doctor getting a copy of the book signed by her and asking for confirmation that Joan had been happy, Verity replying in the affirmative before The Doctor left.

Although the similarity between the two adventures where The Doctor became Smith has prompted some to speculate that the two stories are actually just two different takes on the same experience - the Seventh’s version of ‘Human Nature’ taking place in the pre-Time-War universe while the Tenth’s occurred post-Time-War - given the evident similarities between the two occasions - Smith falling in love with a woman called Joan while a species seeks his Time Lord biodata to increase their lifespan -, the equally obvious differences between the two stories, such as the nature and motive of the enemies seeking The Doctor’s Time Lord DNA and the circumstances under which The Doctor became human in the first place, make it more likely that the similarities are just a coincidence (Much like the Third and Fifth Doctors defeated an alien representation of the devil by blowing up a church in "The Dæmons" and "The Awakening"; neither story was connected to the other beyond The Doctor’s involvement in both, but there were nevertheless intriguing similarities between the two threats).