The Ninth Doctor regenerated into the Tenth due to cellular damage caused by absorbing the energies of the time vortex at the climax of "The Parting of the Ways". In the Children in Need mini-episode, the Doctor initially exhibited stable behaviour as he introduced his new form to Rose Tyler, showing particular interest in his appearance; but soon began acting erratically and said that his regeneration had "gone wrong". He remained in a delirious or comatose state through most of the events of "The Christmas Invasion" until his regeneration was settled through absorbing the free radicals and tannin from some hot tea that had dripped onto a power source inside the TARDIS. He then saved the Earth from invasion by defeating the leader of the alien Sycorax using a satsuma.
The Tenth Doctor and Rose went on to rescue Queen Victoria from a werewolf. The Doctor was knighted as "Sir Doctor of TARDIS" as a reward — a title he later used during his first journey with Martha Jones — although Victoria banished them from the British Empire and set up the Torchwood Institute to defend Britain from paranormal threats and wait for the Doctor's return. He would finally encounter the Institute in "Army of Ghosts".
In "The Girl in the Fireplace", he appeared to develop romantic feelings for Madame de Pompadour while attempting to discover why clockwork androids on a 51st Century spaceship were stalking her throughout her life. Ultimately, he was unable to take her with him as the last, asynchronous time window returned him to her after her death.
In "Rise of the Cybermen", the Doctor was forced to fix the broken TARDIS by giving up the energy from ten years of his life (but he claims that it is worth every second). Given the general longevity of the Time Lords, whether this will affect him to any significant degree is yet to be seen, although it could also be interpreted as affecting his biological age (the Sixth Doctor said in Attack of the Cybermen that a few decades were "a handful of heartbeats for a Time Lord").
In "Blink" the Doctor and Martha were temporarily trapped in 1969. It's not shown how long they were there but Martha had to work in a shop to support him, indicating they were there for at least many days.
In "The Doctor's Daughter," the TARDIS took the Doctor, Martha, and Donna Noble to the planet Messaline in an unspecified time period. Armed human colonists forcefully extrapolated the Doctor's DNA and created a young female soldier for their army. She later chose the name "Jenny," as suggested by Donna, alluding to her status as a "generated anomaly". Despite being reminded of the loss of his family and his former status as a parent, the Doctor eventually accepted her as his 'daughter', only to be separated from her by the end of the episode after he believed her to have died, not knowing that she later returned to life.
The Tenth Doctor has used his psychic abilities more often on screen than his previous incarnations. He continues to use the Ninth Doctor's psychic paper, but has also been seen using telepathic techniques several times (for instance, in "The Girl in the Fireplace" and "The Shakespeare Code"). In "Last of the Time Lords", he uses his telepathic skills over a year to tap into the Archangel satellite network to rejuvenate himself with humanity's belief in him. He is then able to manipulate the combined energy apparently created by that belief, using it alternately as a shield and as a weapon, in the form of telekinesis.
In "Voyage of the Damned" the Tenth Doctor states that he is 903 years old. This contradicts previous statements about the Doctor's age, as the Seventh Doctor states his age as 953 in the episode Time and the Rani, though this follows the idea, started by the Second Doctor in The Tomb of the Cybermen, that the Doctor is not very good at keeping track of how old he is.
The Tenth Doctor "inherited" Rose Tyler as his companion, who left in "Doomsday", the final episode of the 2006 series. At the end of the same episode, a bride named Donna Noble, played by Catherine Tate, appeared in the TARDIS as a result of her Huon particle intake, and appeared in the 2006 Christmas special, "The Runaway Bride". In the episode's dénouement, she refused his offer of full-time companionship, instead suggesting he find someone else. She did, however, return as the full time companion for Series 4. Rose is also set to return for the three finale episodes in Series 4, and has already made three foreshadowing appearances, none of which the Doctor witnessed.
Rose's boyfriend, Mickey Smith (Noel Clarke), a recurring character from the previous season, joined the TARDIS crew as a regular companion in "School Reunion". Mickey departed the TARDIS in "The Age of Steel", replacing his deceased counterpart on a parallel Earth. He returned and departed for good in the 2006 two-part series finale, "Army of Ghosts" and "Doomsday".
Rose's mother, Jackie Tyler, was also a recurring character in the first two series, and played a major role in several episodes, notably "The Parting of the Ways", "The Christmas Invasion" and "Love & Monsters", finally travelling in the TARDIS by accident in "Army of Ghosts". Though she travelled in the TARDIS with the Doctor she is not necessarily considered a companion.
In series three the Doctor was joined by a new companion called Martha Jones, played by Freema Agyeman.Agyeman previously played Adeola in "Army of Ghosts", a character who died during the course of the episode, later revealed to be Martha's cousin. Martha joins the Doctor after they save each other's lives during a Plasmavore attack, and the Doctor offers her a single trip in the TARDIS by way of thanks. She continues as his companion following a trip to the Globe Theatre, and was made an "official" companion — by receiving a key to the TARDIS — in "42". Martha returns to Earth to finish her medical training in "Last of the Time Lords", but leaves her superphone with the Doctor so she can call him if she wants to come back, which she is set to do in 5 episodes of the fourth series, teaming up with the Doctor and Donna as they battle Sontarans and returning later for the finale. An "in-between" guest stint in the Doctor Who spin-off series Torchwood revealed that Martha had qualified as a doctor and now works for the Doctor's one-time employers, UNIT.
Previous companion "Captain" Jack Harkness was originally to have rejoined the TARDIS crew in the 2006 series. However, this plan was abandoned, in part because of Harkness' role in Torchwood. It was initially announced that there would be no crossovers between the two series, but Harkness returned to Doctor Who in "Utopia" for the final three episodes of the 2007 series. In "Last of the Time Lords", the Doctor re-offers Jack full-time companionship but the events of the episode cause Jack to realise that his friends in Cardiff need him, declining the offer he had pined for.
Although she is not considered an official companion, the character of Sally Sparrow in the 2007 episode "Blink" fulfils many of the functions of a companion in this episode, in which she is "recruited" by the Doctor to rescue him from 1969; the episode focuses on her as she follows clues left for her by the Doctor and several allies throughout time, before she actually meets the Doctor at the story's conclusion. The episode "The Girl in the Fireplace" has the Doctor offering Madame de Pompadour the chance to become a companion (if briefly), but circumstances render this impossible and she dies before being able to take him up on the offer. After Martha's departure, pop star Kylie Minogue appeared in the 2007 Christmas special, "Voyage of the Damned", playing a character named Astrid Peth, 'a one-off companion' for the episode. Both she, and later, Jenny, the Doctor's 'daughter' by cloning, accepted offers of companionship from him only for circumstances to prevent this too.
In the 2008 series episodes "Silence in the Library"/"Forest of the Dead", the Doctor encounters Professor River Song, a 51st Century archaeologist he will apparently meet in the future and become very dear to him, presumably while he is still in his tenth incarnation. Whether viewers will ultimately learn what relationship the Doctor has with Song is yet to be seen. It was never explicitly stated that she was a companion, but she states that they traveled together often and she gained his complete trust, to the point that she would eventually know the Doctor's true name, something even the Doctor's longest-travelling companion -- the audience -- has never known.
The Tenth Doctor is the first since the Second Doctor to actually say "goodbye" to a companion (specifically, Sarah Jane) rather than simply leaving, or giving some platitude when a companion leaves of their own accord. He has made many mentions of Rose Tyler since her departure to Martha Jones and Donna Noble (and has had references of her made to him), although in past incarnations, he has also made the occasional repeated reference to the likes of his granddaughter Susan and Brigadier Lethbridge-Stewart. At one point he used "Doctor James McCrimmon" as an alias, another reference to a previous companion.
The Tenth Doctor generally displays a light-hearted, talkative, easy-going, witty and somewhat cheeky manner, but combines this with a somewhat egocentric sense of unstoppability when facing his enemies. He is perhaps as ruthless and dangerous as his seventh incarnation ever was, although much less inclined to, or capable of, complex schemes and set goals. This emerged early on when he sent the Sycorax leader (who was attacking him from behind) falling to his death while commenting that, with him, there were "no second chances." In "School Reunion", he acknowledges that he is less merciful than he used to be and has stuck to his "one warning" code, punishing his enemies if they persist in their hostilities. This was most explicitly demonstrated in "The Runaway Bride" when he drowned the Empress of Racnoss' children, and in "The Family of Blood" where he gave each Family member an individual and eternal punishment. In "Forest of the Dead", the Doctor supports his immense self-belief in his abilities and authority in a different way by causing the Vashta Nerada to acquiesce to his ultimatum simply by ordering them to research his personal history. However, in "Partners in Crime", after giving his adversary, Miss Foster, an explicit warning, he tried to save her life at the end of the episode and did not punish her Adipose foster children "because they're children." Donna notes that Martha had been a positive influence on him, citing his infanticide of the Racnoss in their previous encounter. Like his past selves, he is critical of weapons, going as far as to describe people with guns as "the enemy" in The Sontaran Stratagem. His strong personal sense of justice makes him quick to anger when he feels it is violated, as in "New Earth" when he learned of the plague farm run by the Sisters of Plenitude, and after Prime Minister Harriet Jones had given the order to destroy the retreating Sycorax ship, the Doctor warned her that he could "bring down" her government with six words ("Don't you think she looks tired?", whispered to Jones' aide, Alex).
Like the Seventh and Ninth Doctor, the Tenth sometimes uses a cheerful, energetic façade to mask inner emotions. He has a tendency to babble, mixing apparent nonsense with vital information, sometimes acting erratically to put his enemies off guard like some of his earlier incarnations. He can also be rude on occasion, and is not always aware of it, being prone to making comments that to outsiders seem obtuse or rude, sometimes to his own embarrassment. In "The Christmas Invasion" and "Tooth and Claw", he is surprised at his own unintentional rudeness when making disparaging remarks, and Jack Harkness, after reuniting with the Doctor, notes that his "new regeneration (is) kinda cheeky." He has a tendency to use technobabble to describe scientific concepts before substituting it with a simpler, analogous explanation. Further to this, he tends to infantilise names and concepts - his description of non-linear temporal physics as "a big ball of wibbly wobbly, timey wimey stuff" is perhaps the most well known example. He is also able to rapidly switch between moods, from mania to anger to nonchalance and uses this as a form of reverse psychology on several occasions ("Fear Her", "Love & Monsters" and "Army of Ghosts"). In the latter, by switching gears suddenly after failing to dissuade Yvonne Hartman from her current activities, he is able to make her uncertain enough to get his way. Unlike the Ninth Doctor, who showed off his vengeful, rage-filled dark side when up against the Daleks, the Tenth Doctor displayed a more confident, self-assured side when around them, but did not hesitate to taunt them.
It has been made clear that the Doctor is, despite constant interaction with others, a lonely person deep down. In "School Reunion", he describes the ability of Time Lords to live so long as a curse, because while his human companions all someday leave him and eventually die, he continues to live. Other characters have also commented on the Tenth Doctor's loneliness. During a conversation with his nemesis, the Master, he admits that since the end of the Time War and the loss of the other Time Lords, he has been "lonely ever since", viewing the Master's return as the end of this loneliness. Indeed, when the Master subsequently dies, the Doctor openly weeps over his body. While the Ninth Doctor was somewhat standoffish in certain situations, particularly "domestics", the Tenth is more extroverted and gregarious, having quickly established a firmer rapport with Rose Tyler's friends and family than he ever did in his previous incarnation, though his talkativeness sometimes irritates others not used to him. The Tenth Doctor is openly fond of mankind and is apparently in awe of their tenacity and curiosity, a trait previously exhibited by his fourth incarnation. In "The Impossible Planet", he hugs the leader of an Earth expedition for daring to explore a planet orbiting a black hole. In "The Age of Steel", he describes human beings as both brilliant and stupid in the same sentence while arguing the necessity of emotions with the Cyber-Controller. The Doctor even goes so far as to exclaim he's willing to battle the Master across the cosmos as long as he leaves Earth alone in "The Sound of Drums." However, he is also quick to criticise mankind when he feels it is necessary.
The Tenth Doctor and Rose often faced their adventures with a cheerful, almost blasé attitude, even when terror and death happened around them, contrasting his previous selves, who displayed more serious attitudes when in trouble. Queen Victoria commented on this in "Tooth and Claw" when she banished them (as did Agatha Christie to the Doctor alone in "The Unicorn and the Wasp"), and producer Russell T. Davies hinted that there would be consequences to this carefree attitude later in the 2006 series. In "Doomsday", the two were separated seemingly forever when Rose was left in a parallel universe as a consequence of foiling a Dalek and Cyberman invasion of Earth.
The 2006 series continued the exploration of the Doctor's romantic aspects, with the Tenth Doctor sharing kisses with Rose (albeit while she was possessed by Lady Cassandra) and Madame de Pompadour. In "School Reunion", Sarah all but confesses that she had been in love with him. In "Doomsday", during their farewell, Rose tells the Doctor she loves him; he begins to reply but only manages to say her name before the transmission is cut off, leaving him alone in the TARDIS with tears on his cheeks. After this, whenever he is reminded of Rose he sometimes becomes depressed or pensive. In the audio commentary for "Doomsday" the executive producer Julie Gardner claimed that she will confirm to the nation the Doctor was going to 'say it back'. In 2007 episodes, the Doctor learnt that Martha harboured unrequited feelings for him before she left his company - which he inadvertantly inspired by kissing her as a distraction - and also exchanged kisses with Astrid in honour of "an old tradition" from her home planet. Following the complications with Martha (for which he blames himself), the Doctor seems reluctant to embark on any other potentially romantic companionship, and makes sure that before allowing her to join him, Donna understands that all he wants is a friend.
The Tenth Doctor sometimes dons a pair of spectacles, like the Fifth Doctor, whose youthful appearance he shares. In the 2007 Children In Need special, Time Crash, the Tenth Doctor notes other inherited/inspired tendencies when meeting the Fifth Doctor aside from "the brainy specs" (which he observes were worn by the Fifth simply to look clever rather than out of necessity, therefore implying that his are used for the same reason) such as wearing plimsolls/trainers and both of their voices becoming high-pitched when shouting. He also exhibits a remarkable sense of taste, again similar to the Fifth Doctor, ("Planet of Fire"), able to identify the blood type of a blood sample ("The Christmas Invasion") or the presence of mistletoe oil ("Tooth and Claw") just by licking. He also shares the Fifth Doctor's skill with a cricket ball, as demonstrated in "Human Nature". The Tenth also admitted to the Fifth that he was his favourite past incarnation.
The Doctor seemed disappointed that his tenth incarnation was not "ginger", but has worn his own dark brown hair in various ways throughout the series: unstyled in "The Christmas Invasion"; a fifties-style quiff in "The Idiot's Lantern" and flattened forwards in "The Runaway Bride". He wears a dark brown (with blue pinstripes) or a blue (with red pinstripes) suit, a shirt and a tie (otherwise, open-shirted with a light grey t-shirt/vest ("Tooth and Claw", "Planet of the Ood"|), a red-hued t-shirt ( "42") or a black t-shirt ("Midnight")), a light brown overcoat (which he claims was given to him by Janis Joplin), and a pair of trainers, in colours ranging from cream (brown suit), black (tuxedo) or red (blue suit), a costume which Tennant described as "geek chic". The blue suit debuted in Series 3 episode 1, "Smith and Jones", and both suits were worn from his adventures with Martha Jones onwards. According to an interview on Parkinson, David Tennant and Russell T Davies got the idea for the Tenth Doctor's costume from an outfit Jamie Oliver had worn on Parkinson just after David had taken the role. Another part of the Doctor's costume is a pair of dark tortoise-shell rectangular frame glasses; since The Christmas Invasion he has required them in numerous episodes. As noted above, the Tenth Doctor credited the Fifth Doctor with inspiring his trainers and glasses.
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