Fourteenth Doctor

The Fourteenth Doctor was the result of his predecessor's solitary regeneration, having been fatally wounded during her final fight with to save the Earth from destruction. He was mystified by his 'new' body and appearance, given he looked virtually identical to the Tenth Doctor.

A day to come
When the Tenth Doctor encountered the "Vortex Butterfly", he was cryptically told that he would not be "limited" to "thirteen lives." (COMIC: Vortex Butterfies)

The Curator had told the Eleventh Doctor that he might find himself revisiting old faces, “but just the old favourites.” (TV: The Day of the Doctor)

When Captain Lundvik threatened to shoot him, the Twelfth Doctor speculated that he would "keep on regenerating forever" if he was executed. (TV: Kill the Moon) Rassilon would later state his own uncertainty to the number of regenerations the Doctor had been granted, (TV: Hell Bent) with the Saxon Master and Missy also considering their own uncertainty when debating whether to throw the Twelfth Doctor off a roof, believing they "could [be] up and down the stairs all night." (TV: The Doctor Falls)

The Twelfth Doctor was forced through seven false regenerations by a "regeneration vampire". With the assistance of the Eleventh Doctor, the regeneration energy he expended was returned to him. (AUDIO: Regeneration Impossible)

The Thirteenth Doctor remarked to Cleo Proctor that she would stand up again if she "clobber[ed]" her, remarking that she would regenerate "Possibly several times" and then "who can say?" as to how many times. (AUDIO: Salvation) While facing being shot by guards of the Good Doctor, she said any attempt on her life would result in a "miracle". (PROSE: The Good Doctor)

Trying to generate enough energy, the Thirteenth Doctor once considered regenerating and using residual artron energy. (PROSE: The Maze of Doom)

The Thirteenth Doctor wrote a book about all of her lives, also using material written by other incarnations from the past and her fam, to aid the new Doctor if they "[felt] weird" after regeneration. (PROSE: A Short History of Everyone) However, the Doctor found herself in a regeneration-based crisis when, during his plot with the Cybermen and Daleks to defeat her once and for all, made her undergo a forced regeneration into himself to tarnish the Doctor's name. Finding herself trapped in her mind, the Thirteenth Doctor encountered vestiges of her past selves known as the Guardians of the Edge, with a manifestation of the Sixth Doctor remarking that they needed to stop the Master because the Doctor's title and identity was "supposed to be handed over" to whomever "the next one" would be. (TV: The Power of the Doctor)

A familiar body
After the Thirteenth Doctor and her allies foiled the Spy Master's plot and reversed her forced regeneration ‘into him’, the Master decided that if he couldn't be the Doctor, then neither could she. He spitefully exacted his revenge in his dying moments; using his Tissue Compression Eliminator, the Master directed the Qurunx's destructive blast at the Doctor, fatally wounding her. Staving off regeneration long enough to have a final conversation with Yasmin Khan, she then travelled alone to a cliff overlooking the sea, where she could watch the sunrise one last time.

After saying her farewells to her current incarnation, the Doctor began to regenerate; however, the regeneration not only replaced every cell in her body, but, unusually, caused her outfit to morph into a completely different set of clothes. After the regeneration was complete, the new Doctor was taken aback when he realised he possessed a body all-too-familiar; he noted his teeth and new outfit, prompting him to repeatedly exclaim "what?!". (TV: The Power of the Doctor)

First adventures
After getting over his initial shock and settling back into his old body, the Doctor returned to the TARDIS and allowed it to take him to Wembley Stadium during the 1966 World Cup Final, in pursuit of a distress signal. While looking to see if anyone was in distress, and due to some faulty disguises, he found a group of purple aliens in the stadium as time tourists, who were present to watch a historical football tournament when a Dalek flying saucer arrived in the stadium to deploy a force of Bronze Daleks and begin another invasion of Earth. The Doctor got their attention using his sonic screwdriver so they abandoned their attack to attempt to exterminate him instead. Fleeing back to the TARDIS, he encountered a woman who had retrieved the World Cup and offered it to him, believing he was with the police. The Doctor bundled her into the safety of the TARDIS however a Dalek saucer seized it before he could follow, leaving him surrounded by the Daleks. The Supreme Dalek ordered the Daleks to exterminate him and the Doctor closed his eyes as their shots converged on him. However, the blasts had no effect on the Doctor, surprising him as well as the Daleks.

The Doctor was taken aboard the Dalek saucer where the Supreme Dalek demanded to know why the Daleks' weapons have no effect on the Doctor. The Doctor offered to explain, but only after the invasion of Earth was called off. The Supreme Dalek reluctantly ordered the invasion fleet to withdraw from Earth, following which the Doctor explained that the reason the Daleks cannot exterminate him is that they are all facsimiles and are not real at all. The enraged Supreme Dalek refused to accept the idea that none of the Daleks are real, and decided to prove this by having the Dalek fleet fire upon the Earth, apparently destroying the entire planet, much to the Doctor's horror.

The Doctor vowed that if he discovered that the destruction of Earth is real, then he would see that the Daleks would pay. Two armoured figures materialised on the bridge, captured the Doctor and teleported him away, as the Daleks watched in confusion. The bewildered Doctor was surprised to discover his captors are a pair of bipedal gorillas named Claire and Claudine. He was then greeted by the woman with the World Cup trophy, who introduced herself as Georgette, and revealed that the Doctor was really in a theme park called the Dalek Dome.

After having had a look around at the Dome, witnessing a few of its zones, the Doctor returned to Georgette to voice his displeasure at the attraction, deeming it a "tacky, tastless tourist trap" because it made light of the "suffering of billions" for the sake of "a fun day out for the family". When the Chief approached to argue him, the Doctor proceeded to ask questions and was told that the site used to be an animal park until clone safaris were banned. When the Doctor kept asking questions, the Chief got angry and left.

The Doctor figures out the distress signal he was sent came from Georgette. He got mad and attempted to leave the place but Georgette objected that he couldn’t return to the 66-scape until the psychoplasm had vented. Georgette enters a room and leaves the Doctor, sure of his ability to find a way in for himself. At this point, the Doctor can’t help himself but get curious and sonics the door open to find a control room behind the door which holds a sleeping, dreaming, Dalek captive in a tank, designated Specimen Six Sigma. The Doctor then observed as the Dalek woke up. (COMIC: Liberation of the Daleks)

Psychological profile
Upon gaining a body almost completely identical to his tenth incarnation, the Fourteenth Doctor also regained a number of that incarnation’s psychological traits. Indeed, his immediate response to discovering that he had returned to his former body, coupled with the unexplained appearance of a new outfit, led to him repeatedly uttering "what?" in his bewilderment. (TV: The Power of the Doctor)

Nonetheless, the Doctor appeared to settle back into his old body very quickly, with no signs of the post-regenerative trauma that his predecessors suffered. Indeed, his first act post-regeneration was to re-enter his TARDIS, bursting with enthusiasm and curiosity at the distress signal that it had just received. (COMIC: Liberation of the Daleks)

Appearance
The Fourteenth Doctor, upon regeneration, wore a costume distinct from his predecessor's ensemble. His attire included a white button-up shirt, with a tie, a loose fitting tartan waistcoat and trousers. He also wore a navy blue trenchcoat and Converse shoes. He greatly resembled a slightly older version (TV: The Power of the Doctor) of his tenth incarnation, with the Doctor recognising this form by the feeling of his teeth and face. He also spoke in the same accent as his former incarnation. The only noticeable difference between him and his tenth incarnation is that he sported stubble and a slight copper tinge to his hair. (TV: The Parting of the Ways, The Christmas Invasion, et al.)

According to a scan of the sonic screwdriver, the Fourteenth Doctor's kidneys were blue, which he stated was normal. The same scan confirmed he had two hearts and an in tune spleen, thereby confirming all his vitals were normal. (COMIC: Liberation of the Daleks)

Behind the scenes

 * This Doctor's numbered designation was officially confirmed by Russell T Davies in an article posted to the official Doctor Who website following the Centenary Special's broadcast.
 * This in turn makes David Tennant the fourth actor to play multiple separate incarnations of the Doctor, along with Tom Baker who portrayed the Fourth Doctor from TV: Robot to TV: Logopolis and the Curator in TV: The Day of the Doctor, Colin Baker who played the Sixth Doctor from TV: The Caves of Androzani to The Ultimate Foe, another form of the Curator in AUDIO: Stranded 4, and an alternate Fifth Doctor in AUDIO: Doctor of War, and Richard E Grant, who played versions of the Tenth and Ninth Doctors in The Curse of Fatal Death and Scream of the Shalka respectively.
 * Additionally, three actors have played two incarnations of the Doctor at once via regeneration sequences. Trevor Martin played both the Third Doctor and a unique Fourth Doctor in the stage play Doctor Who and the Daleks in Seven Keys to Doomsday, Sylvester McCoy played the Sixth and Seventh Doctors in TV: Time an the Rani, and Paul McGann briefly played the War Doctor at the end of TV: The Night of the Doctor. However, in these cases, the actors were only standing in for these incarnations' primary actors, and they did not play two fully unique incarnations in full.
 * Shortly after his post-regeneration scene in The Power of the Doctor, this incarnation's costume was added as a character-skin in the free multiplayer battle royale video game Fall Guys, released on 1 November 2022.
 * The Fourteenth Doctor is the first incarnation to have their initial post-regeneration story depicted via a comic: Liberation of the Daleks, published in Issue 584-onwards of Doctor Who Magazine.
 * The Fourteenth Doctor is the first incarnation to appear in a Comic Relief sketch prior to their first full television story, and the first to appear in a sketch while a story of any media is still ongoing. This sketch in question was the Red Nose Day sketch Comic Relief 2023, released on 17 March 2023.
 * The sketch saw Lenny Henry, playing the 9.5th Doctor, appearing in his dressing room about to go on stage to host the Comic Relief live-show, when suddenly complaining about stomach pains from eating too much, calling out that he needed "a Doctor". Beginning to flash with regeneration energy, Henry starts to burst into a full on regeneration, suddenly turning into the Fourteenth Doctor who, in a partial re-enactment of his scene in Power, examines himself and exclaims "What?!", then notice he's gonna host the show, looks at his watch and runs off.
 * Previously, the Dead Ringers sketch Tony Blair Regenerates paid homage to the regeneration into the Tenth Doctor at the end of The Parting of the Ways by having Tony Blair, played by Jon Culshaw, regenerate into a new incarnation played by Tennant.
 * The Fourteenth Doctor is the first incarnation since the Second Doctor to emerge from their regeneration with a new outfit. This deviation from the usual norm of the newly regenerated Doctor still donning the clothes worn by their predecessor led to some confusion among viewers. Unlike the Second Doctor, however, the Fourteenth Doctor's outfit change is highlighted via a "burning" effect and a slight electrical sound, and is noted by the Doctor afterwards as he examines his new body.
 * Russell T Davies clarified that "[he] was very certain that [he] didn’t want David to appear in Jodie’s costume." explaining that his reason for the Doctor's clothes changing during regeneration was to avoid stereotyping "the notion of men dressing in "women's clothes", the notion of drag", describing it as a "very delicate" matter, expressing that "it has to be done with immense thought and respect", and that the media would make it "look like mockery" of that culture, as David Tennant is taller than Jodie Whittaker.
 * The idea of the Doctor regenerating into a prior incarnation's likeness was previously proposed by series creator Sydney Newman during a 1986 meeting with BBC One controller Michael Grade, who asked Newman for ways to reformat the programme after Colin Baker was dismissed from the role of the Sixth Doctor. According to the 1996 book Doctor Who – The Eighties, Newman specifically envisioned Patrick Troughton, who previously played the Second Doctor, returning for one season before regenerating into a female incarnation.