Doctor Who thrives on reinventing itself, often coinciding with the change between Doctors or showrunners. In chronological order:
- The show was originally pitched as an educational show, and soon retooled to the scifi story it is today. Originally it was supposed to alternate between historical stories set in the past, and scientific ones set in space (which is why the first two companions were a history teacher and a science teacher). This happens as early as the second serial, which features the Daleks (justified as educational by Verity Lambert because of the anti-war moral), and there's some rather half-hearted forced educational moments in later Season 1 serials (such as a scene where Ian and Barbara pause to discuss Roman means of building arches while on the alien planet Marinus), but by Season 2 even that was abandoned in favour of serving up fun adventure stories.
- The personality of the First Doctor got this a couple of times due to Early Instalment Weirdness. He was originally intended to be a Trickster, the Token Evil Teammate and constantly getting his friends into trouble, with either Ian or Barbara being The Hero depending on the story. However, it was decided that his character was too unlikeable for children, and the Bottle Episode "The Edge of Destruction" was written quickly to serve as his Heel Realization, after which he became a much more helpful and warm character and quite often The Hero. His final Retool was hinted at in "The Aztecs" but hit full-force in "The Reign of Terror", and was for him to become a more comical character whose meddling would often get him into weird trouble with hilarious consequences. All of these alterations were carried over to every other Doctor.
- Every single time the Doctor regenerates into a new actor, it comes with changes of the Doctor's personality and show feel. This is almost certainly the only reason Doctor Who could become the Long Runner that it did. The first regeneration (Hartnell into Troughton) was a relatively small change, but Troughton into Pertwee completely transformed the show's genre, and Pertwee into Tom Baker completely transformed it in a different direction, and again... A basic list, relying on generalisations:
- Hartnell: A grumpy but good-hearted Doctor in an Edutainment Show dealing mostly with historical adventure stories and fairly thoughtful sci-fi.
- Troughton: A comical, silly and straightforwardly heroic Doctor. Show dropped the historical adventures and began focusing exclusively on aliens. The stereotypical Troughton story is a "base under siege" where a small community of scientists are trying to do something important while some sort of rubber suit monster and/or foam is taking over. Six-part adventures become more common than the four-parters that were the main format of the Hartnell era, but the serial length is still very irregular.
- Pertwee: A serious, charming aesthete Action Hero Doctor in a Tuxedo and Martini-Monster of the Week genre mashup show (after all three regulars including the current Doctor decided to leave at the same time). The Doctor has a whole new backstory and has been exiled to Earth. The show takes on a much stronger Monster of the Week format due to said exile — Hartnell and Troughton would often have general adventure stories with no clear monster figure and themes of exploration or dealing with the culture of a Planet of Hats, but Pertwee got to fight a new ridiculous Killer Whatever alien (and/or, later, his Evil Counterpart) every month without fail. Companion role goes from being a small group of mixed-sex travelling companions to a primary young, female partner (Liz, Jo, Sarah Jane) and a larger pool of UNIT coworkers who can be drawn in and out of stories as needed. Serial length standardises as six-parters with a four-parter Once a Season. And everything's suddenly in colour! The most extreme retool to date, it was so successful that the format hung around for most of Tom Baker's tenure.
- Tom Baker: A slightly byronic Manchild Doctor with funny mannerisms. Show drops the secret agent elements but keeps the Monster of the Week formula, very rarely dealing with recurring enemies (only two Dalek stories in the whole seven-year tenure). Show also begins taking heavy influences from Hammer Horror films, giving it a gothic tone and Genre Shifting the show into "horror" rather than sci-fi, though this is dropped in favour of comedy and pure sci-fi later after a Moral Guardian crackdown. The final season shows a sudden increase in production values, the theme music and visuals are changed, the Doctor starts wearing a more costume-y outfit in order to provide more visual identity and the writing gets dark again, though staying out of horror for the most part. Companion role is streamlined, dropping the coworkers for simplicity and not bothering with male companions for Cast Speciation reasons, focusing on a single young, attractive female character.
- The Fourth Doctor's personality went (inorganically) through three different archetypes depending on who was producing his run, as each writer envisioned a different tone for the show. His first personality, during his Gothic Horror-influenced Philip Hinchcliffe era, was a Creepy Cute Nightmare Fetishist Manchild who occasionally struggled with big moral decisions but was also very unpredictable, possessing Blue-and-Orange Morality. His second personality, during Graham Williams' Lighter and Softer-cum-Denser and Wackier era, was Played for Laughs — much more of a Cloudcuckoolander and also much more of a Nice Guy (dipping into Fun Personified at times), although also more of an Attention Whore and more awkward to deal with. His third personality, when the show decided to get Darker and Edgier again under John Nathan-Turner, made him philosophical, Byronic, dignified and quite morbid, developing some Chessmaster qualities and dispensing with a lot of the childlike aspects of his character. Broadly, his taste in hat colour indicates which personality he's going through at the time — a brown hat for the Gothic Horror era, a green hat for the Lighter and Softer era, and a red hat for the Byronic era.
- Davison: A subtle and human Doctor to contract with the previous Large Ham Doctor, whose sonic screwdriver is destroyed as a symbolic attack on some of the lazier Tom Baker writing. Show adds Soap Opera elements like a large rotating cast of companions and a bigger focus on the Doctor as a vulnerable and emotional figure (where Tom Baker had verged on Invincible Hero) and even going into twice-weekly soap-like scheduling for a time. Horror is back on the table and Wham Episodes and Continuity Porn are the name of the game (including a whole season of recurring monsters in the run-up to the anniversary special), and a companion gets killed off for the first time since William Hartnell.
- Colin Baker: An attempted Character Check, much Darker and Edgier Doctor; arrogant, violent and verbose. Stories attempt Deconstructions of standard Doctor stories and begin to incorporate some engagement with the implications of time travel itself (which is usually just used as a device to get the Doctor into wherever the adventure will happen), like Future Me Scares Me and Timey-Wimey Ball. More stylised, crazy sets and costumes occur. The format changes to 45-minute episodes, but switches back to 25-minute ones after a BBC-imposed 18-month hiatus.
- McCoy: A particularly impossibly wise Doctor who is sociopathic and manipulative, who also happens to be a cheery, funny vaudevillian. After some Early Instalment Weirdness, starts focusing strongly on the companion's home life and personality, as well as continuing the trend of deconstructing the Doctor's relationship with the companion. Themes of social justice also crop up much more often than before, with the show tackling topics such as racism, social isolation, and Social Darwinism; one story is even a less-than-subtle critique of the Margaret Thatcher administration and homophobia in British society. The story moves in a more arc-based direction, but the show is cancelled here before the overarching plotline can be resolved.
- McGann: A rather naïve and enthusiastic Doctor, aimed at being an Adaptation Distillation of traits from the popular Fourth Doctor. Show begins taking heavy influence from The X-Files, and is the first to introduce explicit romance between the Doctor and his companion (although Tom Baker and Pertwee had both dabbled in Ship Tease). Focus remains on the human companion rather than on the Doctor. (The Expanded Universe version of him changes a great deal from this, though.)
- Richard E Grant: Gallifrey has been destroyed and the Doctor has a whole new truckload of Angst to be a Stepford Smiler about, and the Master gets to become the Doctor's boyfriend with a Heel–Face Turn. Pertwee-era-Pastiche story, and the companion's personal life continues to be important, but the Doctor contains most of the focus. Mythology Gags run thick. Also, it's Web Animation. Retconned out when a new live action series got a go-ahead.
- Eccleston: Gallifrey has been destroyed in the Time War and the Doctor has a whole new truckload of Angst to feel about it. The focus remains on the human companion, and a whole lot of Soap Opera elements are explicitly added to genre mashup levels. Since production values are notably better the monsters tend to be quite self-consciously campy, and the science fiction elements are intentionally very soft. The Doctor is also portrayed as almost always knowing the monsters he's up against, a new development. However, the mythology of the series is deliberately shied away from in order to bring in new fans. Also, we focus on time travel rather than space travel; when we leave Earth it's for an Earth space station in the future or the planet humans migrated to and still call Earth in the further future. That part continues into the Tennant era, changing only with series four. There's a theme of humanity's journey that runs through Nine and Ten's years.
- Tennant: Much more romantic and gentle than Eccleston's Doctor, but also much more brutal and ruthless (from his first episode: "No second chances. I'm that sort of a man."). Feel of the show remains much the same as with Eccleston due to his extremely short tenure, but older continuity slowly starts to resurface and the series takes the focus back onto the Doctor rather than on the companions while dealing with constant themes of morality, loneliness and Shooting The Dog. First Doctor whose seasons turn out to constitute a Myth Arc in the end.
- Smith: Very similar to Tennant's Doctor, but somewhat less moody and simultaneously more childish and more world weary. With no prior cast members involved and a new showrunner, the mood changes to a "cosmic fairy tale" theme with a much stronger focus on Time Paradoxes than ever before, and incorporating strong elements of Sex Comedy. Horror episodes are significantly more common during this period, and Mythology Gags to the Classic series, purposefully avoided or kept subtle before, become thick on the ground. We also see more alien worlds — they're still Human Aliens, of course.
- Capaldi: Another Character Check Doctor: A bonkers Pragmatic Hero marked by his Character Development from Grumpy Old Man and Deadpan Snarker questioning his morality (Series 8) to Cool Old Guy in a partnership of equals with a long-tenured companion (Series 9) to professor/grandfather figure to a new one (Series 10). Most of the romantic hero elements from the previous few Doctors are gone. Series 8 has him take more of a backseat to his companion than he'd done previously in the new series (with her being retooled from her debut in Smith's final season), but he returns to the show's center afterward. Horror episodes and grisly content are common as the tone becomes melancholy and mature. The Doctor spends more time dealing with the fallout of his actions, being chewed out by others, and struggling with his needs for companionship and a Morality Chain when he will never have anyone forever than he used to. Human companions have lives outside of the TARDIS, and the first non-human companion of the revival is introduced. Pacing is notably slower, especially in Series 9, which is mostly multi-part stories. With this Doctor a throwback to those of the Classic Series (especially One, Three, Four, and Six), Continuity Porn runs rampant. Like the Pertwee, Eccleston, and Tennant eras, mostly confined to Earth and human outposts.
- Whittaker: A much more idealistic figure than most of the new series Doctors at first, with emphasis on her inspirational nature and belief in the good in all sentient creatures, but also more emotionally distant from her companions than before (Series 11), then as she struggles with emotional trauma, she becomes much moodier and emotionally withdrawn, more willing to let her anger show (Series 12). Maniacal alien villains become less common in favor of Obliviously Evil villains, Well-Intentioned Extremist types, and Humans Are the Real Monsters messages. Social justice, while occasionally showing up in the earlier series, becomes much more of a focus for the first time since the Seventh Doctor's tenure, with episodes critiquing racism, religious bigotry, laissez-faire capitalism, and sexism. While starting off much Lighter and Softer, there are still several horror episodes. Continuity is significantly reduced in Series 11 as a way to gain new fans, with no returning characters, but makes a big return from Series 12 onwards, with the show's mythology key to its Story Arc (see below).
- The all-but-abandonment of "pure historicals" (episodes with no science fiction elements other than the Doctor and his TARDIS himself) and their replacement with much rarer "pseudohistoricals" (stories where the Doctor will travel back in time to fight an alien) was an early attempt at this, as historicals were proving much less popular with audiences than the People in Rubber Suits. The first story to contain pseudohistorical elements was "The Chase" (which contained a short Wacky Wayside Tribe sequence involving Daleks on the Mary Celeste), and the first true pseudohistorical was "The Time Meddler" — the reveal that it was another time traveller causing the mayhem was a shocking twist to an audience expecting a Costume Drama with Horny Vikings, but pretty much expected now. The last pure historical of this era was "The Highlanders", although the Peter Davison story "Black Orchid" also fits the strict definition of this story type.
- The wiping-out-from-all-of-existence of the Time Lords between the show's 1989 cancellation and its 2005 resurrection might also be considered a retool. Actually, the introduction of the Time Lords counts as a bit of retool in itself. Originally the Doctor simply came from a mysterious, seemingly human-based civilization, with no more details offered.
- The restoration of Gallifrey in the 2013 50th Anniversary Episode is also one of these. It almost immediately preceded the Eleventh Doctor's regeneration episode, and helped end a Story Arc that had been pushed as long as it could have been and now only served to hold the character back.
- Series 12 in 2020, however, introduced a Story Arc that destroyed Gallifrey and the Time Lords again and revealed in the Season Finale that the title character is not a native Gallifreyan but an infinitely-regenerating "Timeless Child" from an unknown world, who became the progenitor of the Time Lord race against their will and lived numerous lives prior to becoming the First Doctor. With this came the introduction of the first proto-Doctor, played by Jo Martin as the second female and first non-Caucasian incarnation of the character to be explicitly identified as such.
