Fourteenth Doctor

Energetic, curious and driven, the Fourteenth Doctor was the result of his predecessor's solitary regeneration after being mortally wounded saving Earth once more. This incarnation of the Doctor curiously looked virtually identical to a previous form, the Tenth Doctor, a fact which left the Doctor mystified over his 'new' body and appearance.

A day to come
The Curator had told the Eleventh Doctor that he might find himself revisiting old faces, "but just the old favourites." (TV: )

After learning about her coming regeneration, (TV: ) 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: ) 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: )

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 skinny man with sideburns was back. (PROSE: )

First adventures
After getting over his initial shock and returning to the TARDIS, (COMIC:, PROSE: ) the Doctor continued to question his old appearance, particularly his teeth which belonged to himself a few regenerations back, before beginning to write a letter to the readers of an annual, reminiscing about a whole year having passed since he last wrote, reminding himself that this was the idea with annuals.

Describing his own year as having "its ups and downs" — recollecting his recent, beautiful regeneration on a clifftop with the sunrise and arms stretched out — he wrote that he was aware that this regeneration was different, given his resemblance to a prior incarnation, but realising he did have new braincells due to remembering the custard-cream dispenser he had forgotten about. He also noted his hands, which he hadn't realised he had missed up until then, dismissing the possibility he would lose them again in a swordfight with a Sycorax; he described other familiar features of himself, from his "fantastic" hair to his "slightly quizzical left eyebrow" to his "brilliant" grin, concluding that it was nice to have this face back still questioned why.

The Doctor also made noted about his clothes having changed too, which hadn't happened in a very long time, but was distracted by something "making a right old racket" on the opposite side of the console. (PROSE: )

Seeing that the noise was a distress signal, the Doctor allowed the TARDIS 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, causing the Daleks to abandon their attack in an 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, only for a Dalek saucer to seize 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 could not exterminate him was that they were all simulacra of real Daleks. The enraged Supreme Dalek refused to accept the idea that none of the Daleks were 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 was 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", angering its Chief. The Doctor attempted to leave the place but Georgette objected that he couldn't return to the 66-scape until the psychoplasm had vented. He subsequently followed Georgette to a control room behind the door which finding sleeping, dreaming, Dalek captives in a tanks. One, designated Specimen Six Sigma, woke up which the Doctor realised was likely due to his questioning of the Daleks’ existence in the 1966 simulation, prompting Georgette to shut it down, depositing the TARDIS in the lobby. He rushed to the TARDIS where they discovered Georgette's simulacra was still functional and had been forced to let the Supreme into the ship to survive the shutdown.

more to be added (COMIC: )

Further adventures
At some point, he got a new sonic screwdriver. (WC:, PROSE: )

At one point, the Master attempted to force every Doctor to regenerate at once, getting their faces muddled up, with the Fourteenth Doctor's being mixed with his twelfth, fifth, and eighth incarnations. A human was tasked with identifying each Doctor to repair the timeline. (GAME: )

The Doctor asked a human to examine two pictures of a man following a rupture in time that resulted in the creation of multiple versions of reality. (GAME: )

Death
The Fourteenth Doctor was later succeeded by his fifteenth incarnation. (PROSE: )

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: )

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 (COMIC: ) while writing a letter (PROSE: ) and at the distress signal that it had just received. (COMIC: )

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 brown tartan waistcoat and matching trousers. He also wore a navy blue trenchcoat and Converse shoes. He greatly resembled a slightly older version (TV: ) of his tenth incarnation; (TV:, etc.) the Fourteenth Doctor identified his similar teeth, his hands he hadn't realised he had missed, familiar face, fantastic hair, a "slightly quizzical left eyebrow", and "that brilliant grin". (PROSE: ) He also regained his sideburns. (PROSE: ) He initially recognised this form by running his tongue over his teeth (PROSE: ) and feeling his face with his hands. 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:, , etc.)

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: ) He also had different braincells upon regeneration. (PROSE: )

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 first actor to portray two seperate mainline incarnations of the Doctor and the fourth actor to play multiple separate incarnations of the Doctor, along with Tom Baker who portrayed the Fourth Doctor from Robot to Logopolis and the Curator in The Day of the Doctor, Colin Baker who played the Sixth Doctor from The Caves of Androzani to The Ultimate Foe, another form of the Curator in Stranded 4, and an alternate Fifth Doctor in 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 Time and the Rani, and Paul McGann briefly played the War Doctor at the end of 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 short story A Letter from the Doctor, set just after the first panel of Liberation, also was therefore part of his post-regeneration, with the final line of the short story being the same first line spoken by Fourteen in the comic.
 * 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.