It all started out as a mild curiosity in a junkyard and now it's turned out to be quite a great spirit of adventure.
— The Doctor
The longest running science-fiction series in the world, first airing on BBC TV on 23rd November 1963. It takes place in and established the Whoniverse, which has a constantly adapting continuity. It also spawned the truly vast Doctor Who Expanded Universe.The premise of the show is simple enough: it follows the adventures of a renegade Time Lord, the Doctor, and his various companions through time and space. Travelling in his living and sentient time machine, the TARDIS, he meets many foes, ranging from heavily armoured robots to killer microbes and pollen to — well, members of his own race. Part of the longevity of the series is that when an actor leaves, the show gets around this by killing his character off, only to "regenerate" the Doctor into a new form played by someone else (sometimes by someone significantly older or younger). As a result, the same character has appeared in the series from the beginning, but his new personality and new tastes give a show a distinctly different atmosphere with each regeneration.The show originally ran from 1963 to 1989 (with an 18 month hiatus in 1985-6 caused by Executive Meddling, during which it "rested"). A Made-for-TV Movie aired in 1996, in which the Seventh Doctor regenerated into the Eighth. This was created as a pilot for a revival, but nothing else resulted. Between 2001 and 2003, The BBC produced a series of webcasts which it considered in every way an official continuation of the series (insofar as the Beeb ever indicates what is and isn't canon). It is possible more would have been made but for a very exciting development on the television front: in 2005, a Revival began. It's a direct continuation of the old series, rather than being a Continuity Reboot, with the Ninth Doctor being a direct successor to the original series incarnations. The revival series has radically upgraded production values (the old show is known for its often rubbery monster prosthetics and bad Chroma Key), shorter story arcs but much more continuity throughout, and it introduced deeper Character Development and romance to the series. As such, 2013 marked the show's 50th anniversary. The original series lasted 26 "seasons", while the new annual runs of episodes are called "series": officially, the show went from Season 26 to Series 1, and so on.Doctor Who is a British institution and considered a key part of British culture: even the Queen is a fan. In 2013, to mark the show's 50th anniversary, the Royal Mail honoured the show with a set of 12 stamps — one for each Doctor, and the last featuring the show's villains.The original 1963-89 episodes are now considered such an important part of the BBC's home video output that they have their own freelance Restoration Team, devoted to restoring and remastering vintage episodes to as much of their former glory as possible. In the process they have pioneered a number of brand new restoration techniques, with names like Reverse Standards Conversion, Chroma Dot Colour Recovery and Vid FIRE, which can then be applied to other vintage TV shows.The show has spawned several spinoffs within its canon Whoniverse, which have occasionally crossed over with the main series. Except where noted, these take place in (then-) present day Earth.
K-9 and Company (1981), a failed Pilot Episode with the Doctor's former companions Sarah Jane Smith and K-9. The episode aired as a Christmas special in 1981.
K9 (2010- ?), an Australian children's show (produced by a company other than the BBC) which continues in the spirit of The Sarah Jane Adventures. It takes place in a late 21st century Dystopian London.
There are additionally many adventures in almost all types of media, often made by the cast and crew of the TV series. Collectively, these are known as the Doctor Who Expanded Universe, with varying levels of canonicity towards the Whoniverse. The Expanded Universe generally follows the TV series canon, but the TV series is free to ignore the all different branches and mutually contradicting timelines of the Expanded Universe. (The BBC never comments on their canonicity, causing a fair amount of debate and Epileptic Trees.) Stories outside the TV series canon tend to be Darker and Edgier, and often tackle themes that the TV series can't dive into for any reason, as well as story ideas that were proposed but simply never developed for television. The most prominent of these spinoffs are the ongoing Big Finish Doctor Who audio stories by Nicholas Briggs, starring all the original actors. Quite a few stories from the Expanded Universe ended up adapted for the revived TV series.The series also had a behind-the-scenes Companion Show called Doctor Who Confidential which aired in 2005 and was canceled in 2011.For more detailed information, check the Analysis tab.
Alan Fridge: Since the revival, a lot of tabloid stories have claimed exclusives on upcoming plots. It's a very scatter-gun result. It helped that the last minute or two of the penultimate episode and the entirety of the finales were withheld from press previews.
Alien Invasion: Both types, almost constantly. In some cases, it's not necessarily Earth that the aliens want to invade, nor is the species invaded human at all. In many future-based stories, humans are themselves the invaders. We're usually not outright malicious, but we're often quite destructive to native species, paralleling historical imperialism and colonization.
Foremost on the list are the Daleks; super-intelligent, genetically engineered, Always Chaotic Evilspace Nazis encased in non-humanoid Powered Armor. They were designed to feel no emotions other than hatred, prejudice, anger and cruelty. They also experience fear but try not to show it. They are utterly fanatical about their own inherent superiority, to the point where civil wars have broken out amongst them if factions start displaying minor differences, and have chosen death when "contaiminated" by foreign DNA. Their goal is nothing less than to ''ex-term-i-nate'' all life in the universe (and, once, the multiverse). They often tend to find themselves dealing with Earth, which they hate. (But what don't they hate?)
The Cybermen were alternate humans, from Earth's twin planet Mondas who, in an example of Cybernetics Eat Your Soul, converted themselves into emotionless cyborgs. They are exclusively concerned with the survival of their race, and the best way to do that is to forcibly convert humanity into them. That they are a direct threat to mankind means that they have also sought to destroy them, or sizeable chunks, in the distant future when we manage to successfully fight back.
The Time Lords were a race of supposed non-interventionists, who were (until the Last Great Time War), in the worst interpretation (Depending on the Writer) really a controlling, elitist and stagnant, race of Sufficiently Advanced Aliens who, as their name suggests, had mastered Time Travel, amongst other technologies. They had decent members, notably the Doctor himself, but they throw up plenty of maniacs like the Master and the Rani, not to mention their insane founders Rassilon and Omega.. As the Time War drew to a bloody close they became a race of Omnicidal Maniacs who were ready to put an end to time itself in an effort to avoid ultimate defeat, which means they last showed up as a villain race.
Aliens of London: The Doctor speaks with an accent. Which accent depends on the incarnation. The original series Doctors mostly tended towards Received Pronunciation, Seven sounded Scottish, Eight sounded Liverpudlian, Nine Mancunian, Ten had the accent of Estuary London (as opposed to the actor's native Scottish accent), and Eleven has a Northampton accent.
Other examples, such as the Daleks, the Slitheen and Matron Cofelia are explicitly speaking English (whether they've learned English or are using different Translator Microbes are never made certain; the Daleks, however, have been demonstrated speaking different languages when appropriate, such as German in Germany). The Judoon in "Smith and Jones" are shown using alien technology to learn English in moments, after initially speaking their own alien language.
there is a Who episode from Tom Baker days where aliens, seeking to conquer 17th Century Earth, communicate with their human servants via mind-to-mind telepathy. Unfortunately the aliens are frustrated by the fact human brains and telepathy do not go together too well, and the communication is full of static along with low-IQ moments on the parrt of the humans, who can't grasp that they really have to focus to be able to transmit and receive...
All Myths Are True: And they're all aliens. Vampires, werewolves, witches, ghosts, the yeti, the Loch Ness Monster; even the devil is an alien.
The Alleged Car: More like "The Alleged TARDIS" though it's in even worse shape by the 2005 series. He's had it for several centuries, and it was already ancient when he got it, yet it's still immensely powerful and advanced, even by alien standards. In "The Eleventh Hour" however, the TARDIS regenerated along with the Doctor and seems to be in a bit better shape. Two people (rather than the designed six) piloting it, as well as leaving the handbrake on doesn't improve its poor state and Explosive Instrumentation.
Alternate Landmark History: It boasts a wide collection of 'explanations'; enough to warrant its own section on the page as well as providing the page quote. The throwaway lines are examples.
Alternate Universe: Oddly enough, not extensively used. There are alternate universes in the Who multiverse—one Classic Series Story Arc took place in one called "E-Space" and the story Inferno has a Mirror Universe, and the Russell T Davies era has at least two, a Zeppelins from Another World universe and an alternate timeline world centered on Donna Noble in "Turn Left"—but travel between alternate universes seems to be extremely difficult (compared to travel in time and space, creating and controlling a black star, making dimensionally transcendental ships...) and very dangerous.
Although the Doctor states that it used to be easy to do before the Time War; since then, though, the universe(s) don't seem to like letting the travel occur.
The Doctor, particularly Ten, puts his companions (who are usually, if not always, young and female) before anyone else. Also the new Doctor Who series suggests that his companions represent his humanity in a universe full of mass death, as seen in "The Fires of Pompeii" when Donna convinces him to go back for one family among all those destroyed in Pompeii. It's mentioned some times that he feels responsible for them because it's his fault that they are in danger, since he brought them to wherever it is they are. The Doctor has sacrificed two of his regenerations for a girl now.
Then you have Rory:
The Doctor:All of creation has just been wiped from the sky. D'you know how many lives have now never happened, all the people who never lived? Your girlfriend isn't more important than the whole universe. Rory:(punches him) SHE IS TO ME!
Amy and River are gender-inverted examples. River will rip the entire universe apart in order for the Doctor not to be killed, and Rory is the only thing that convinces older!Amy to defy all laws of time.
Ancient Astronauts: Earth has been visited a lot over its history. At this point, it's hard to think of a religion that the series hasn't had aliens meddling in at some point.
Animate Inanimate Object: The Nestene Autons and the Weeping Angels. The Weeping Angels are a strange example; they only move when no one is looking at them (except for that one occasion in "Flesh and Stone"), making it more like short range Offscreen Teleportation.
Anyone Can Die: Unless someone happens to be a historical figure (and eventhen...), there is a good chance they will die before the end of the episode. The Doctor and his companions are not immune to death, either.
Apocalypse How: Every point on the scale has either been threatened or carried out by now. Especially X and X-2. Xs happen almost once a season, multiple times in a few episodes. The Time War managed to pull an X-3. The revival's season finales are usually a threatened or averted X-4. There have been two Class Z Threats in the modern series. Not to mention a Z-3. Creation got better.
Arc Symbol: The crack in series five of the revived series, which appears at least Once an Episode. Notably, it is exactly the same shape everywhere it appears.
Series 3 had various mentions of a Mr Saxon, evidently the a public office figure running for Prime Minister. Who is later revealed to be the Master.
Series 4 was chock full of them, tied together by a theme of things disappearing. First there was talk of vanishing bees, then planets, then moons, and finally Wilfred's "the stars are going out." This all turned out to be due to the Daleks and their planet-stealing operation. Occasionally the phrase "Medusa Cascade" appeared as well; the season finale ended up taking place there.
Unusually, Series 4 also had arc words specific to each of the two main characters. The Doctor had "she is returning," while Donna Noble had the thoroughly unsettling "there's something on your back." In addition to that, The Doctor also referred to The Shadow Proclamation on several occasions before it was properly revealed.
Series 5 had "cracks" which is spoken often but also emphasized visually. Near the end most episodes, after the Doctor had left, the camera would linger on some part of the scenery where a mysterious crack similar to the one that appeared in the first episode of the series had appeared, later revealed to be a result of the TARDIS exploding on June 26 2010. The cracks played a more prominent role in some episodes than others.
The Moffat era has its own arc words: "The Pandorica will open" and "Silence will fall."
Expanded upon in series 6 to "Silence will fall when the question is asked."
Series six adds "Tick-Tock goes the clock." It's the first verse of a recurring nursery rhyme. The second verse changes each time but always refers to the Doctor's impending death.
Perhaps the biggest arc words spanning the whole series is: "Doctor Who?"
It is also the question that will be asked, the question that has been hiding in plain sight, where "Silence will fall."
Series 7 has "Run, you clever boy. And remember." They're the last words of Clara Oswin Oswald, who has died twice in two different lives, each time with the same last words. Trailers aired after the 2012 Christmas special and in March 2013 reveal they will be invoked at least twice more in Series 7.
Badass Normal: Quite a lot of the companions fall into this, particularly since the 2005 reboot.
Jenny Flint, a Victorian chambermaid who ends up married to Vastra, an ancient Siluriuan lizard woman, and masters hand-to-hand combat, sword fighting, and the art of wearing skin-tight leather armour under her maid's uniform.
Twice in series 5, the freakin' Daleks pull one on the Doctor.
First, in "Victory of the Daleks", they let him declare himself as the Doctor and identified his enemies. This was exactly what the Daleks wanted, as their Progenator wouldn't recognize their spoiled DNA. They needed their oldest and most powerful enemy to tell the Progenator who they were, setting off the creation of a new bigger, badder, and technicolor Dalek race. Nice Job Breaking It, Hero...
Then, in "The Pandorica Opens", they let the Doctor fall straight into the Pandorica, supposedly trapping him for good.
The Seventh Doctor is a master Chessmaster setting up all the pieces and having his enemies and friends effortlessly go where he wants them to go in order to save the day... at first glance. However, many of the TV stories involving this aspect of his character end up revolving around the sudden realisation that something is happening that he didn't actually plan for (such as two factions of Daleks seeking out the Hand of Omega rather than one), or someone does something that he didn't expect, necessitating a frantic run-around as he desperately tries to improvise some stop-gap solution to get things back on track.
Doctor:I don't suppose you've completely ignored my instructions and secretly prepared any Nitro-9, have you? Ace: What if I had? Doctor: And naturally, you wouldn't do anything so insanely dangerous as to carry it around with you, would you? Ace: Of course not. I'm a good girl and do what I'm told. Doctor: Excellent. Blow up that vehicle.
The Tenth Doctor is taken to task by Davros for doing precisely this. Davros points out to the Doctor that he makes a big point of how pacifistic he is, while at the same time manipulatively turning those around him into the kind of people who will blow up their own planet to stop an invasion.
The Tenth Doctor is pretty fond of this — feigning ignorance and getting himself captured so he can be brought face to face with the bad guy of the week. Ninth plays around with it too — "I'm really glad that worked. Those would have been terrible last words."
The Eleventh also pulls one on several characters in Series 6. One of whom was himself. This makes it either extra clever, or cheating, depending on point of view.
Bavarian Fire Drill: The Doctor does this a lot. Psychic paper helps... unless the viewer happens to be psychic enough to see through the illusion, like everyone working for Torchwood, or intelligent enough, like William Shakespeare. Though lies too big will actually break it, as seen in "A Christmas Carol", when it refuses to say he's "widely acknowledged as a mature and responsible adult".
BBC Quarry: Filled in for dozens of planets over the years. Subverted in The Hand of Fear, in which the TARDIS arrives in what turns out to be an actual quarry.
Been There Shaped History: This would take a long time to list, but if it was important, either the Doctor or one of his companions may have caused it.
Belief Makes You Stupid: Religion has rarely come off particularly well in Doctor Who and the series contains many examples of people worshipping things like mad computers or empty spacesuits in the mistaken belief that they were gods.
The first series had the megalomaniac Dalek Emperor and his revived army of Daleks.
The second series kicked off with the equally megalomaniac John Lumic and his version of the Cybermen. The season finale featured the return of Cybermen, now led by a generic Cyber-Leader, but they spend half the time competingwithDalek Sec and the Cult of Skaro.
The third series did show a two-part re-appearance of the Cult of Skaro, but it's ultimately The Master that takes center-stage by the finale.
The fourth season finale had Davros and his resurgent Dalek empire, but Davros is just a representative this time around, while the Supreme Dalek is the one calling the shots.
The Silence, a religious order primarily made up of creepy make-you-forget-they-exist aliens and Large Ham lackeys, are shaping up to be the Big Bad of the Steven Moffat era. They were an unseen man-behind-the-man villain in series 5, causing the cracks in time that almost erased the universe from existence and drove most of that series' villains away from their homes and towards the Doctor. They made their onscreen debut in series 6 with a convoluted and almost-successful assassination attempt on the Doctor, and all indications are that they'll be back in future series of the Moffat era.
In the classic series, the Black Guardian was the Big Bad of two separate story arcs: The Key to Time arc and what was later known as the Black Guardian Trilogy.
Also in the classic series, the Valeyard was the Big Bad of season 23 (The Trial of a Time Lord). Depending on how far you want to stretch the definition, the Master might qualify as such from season 8 and 9, as well.
Bigger on the Inside: The TARDIS is, if not the Trope Namer, then definitely the Trope Codifier, to the extent that the word "Tardis" can apparently be found in some dictionaries defined as "something which appears to be much larger on the inside than on the outside".
The Fourth Doctor: That's because it's dimensionally transcendental. Companion: What does that mean? Doctor: That means it's bigger on the inside than it is on the outside.
Bizarre Alien Biology: Any number of critters, not least of which is the Doctor himself—he can do things like regenerate, re-grow hands, and absorb radiation, transform it into a form harmless to humans, and expel it through his foot. Oh, and he has two hearts.
Bizarre Alien Senses: The Doctor has some kind of "Time Sense" relating to whether or not an event is irrevocably supposed to happen. He has also been shown to be able to slow down his own perception of time. In addition to that, he can taste and smell a host of things that humans can't (such as the blood group in a sample of blood).
Lampshaded in one episode where the psychic paper doesn't work and the Doctor says that it finally found a lie too big to say (namely, that the Doctor was a responsible baby-sitter).
Bloodless Carnage: Usually. A few stories have gotten pretty gory - The Brain of Morbius featured a pretty graphic blood squib when Condo gets shot, and the sheer bloodiness of Season 22 may have contributed to the show's first hiatus. As a family show, the series mostly shies away from graphic violence. Due to the BBC's current attitudes, the show has been far less violent after its return than before its cancellation, though the classic series' violence was often undercut by its endearing phoniness.
A Boy And His TARDIS: The core premise of the show. The Doctor stole his TARDIS from a museum and ran off to see all of everything ever, and save most of it from something else. "The Doctor's Wife" (Nu Who Series 6) tells us that it's just as much "A TARDIS and her Boy", as she claims she stole him by leaving her doors unlocked.
Brainwashed and Crazy: Given how long running the series was it was inevitable this trope would crop up. In fact virtually every Doctor and companion underwent this trope or the milder brainwashed trope at some point in the series as well as guest characters in some stories.
Britain Is Only London: Considering that the TARDIS can travel anywhere and anywhen in the universe, a disproportionate number of episodes in the Russell T Davies era take place in present-day London. (As well as near-future London, 1953 London, 1969 London, 1987 London, Elizabethan London, London in the Blitz and Victorian London.)
Averted in Steven Moffat's first series, series 5, where of the six stories set in the UK, note there's a seventh if you count the Starship UK from "The Beast Below" only two are London-based. Two of series 6's seven stories set or partially set in UK also take place in London, and even then, neither are as the central focus. Moffat's production staff have lampshaded that focusing action on London has started to be a cliché.
Broad Strokes: The series abandons and introduces new concepts and twists on old concepts that were never previously mentioned, and often never mentioned again. Big as it is, the series can get away with this easily.
Camp: The classic series is retrospectively looked at as this, especially the Nathan-Turner years. Davies also deliberately added his own camp moments when he was on the show.
Cardboard Prison: Stormcage, where River is incarcerated. She escapes so often that they go on high alert whenever someone sees her packing. At one point, she phones them to cancel the alert; she's breaking back in, not out.
Cash Cow Franchise: The BBC has been prompt to capitalize on the show's newfound success since it returned. Two decades of Expanded Universe literature and audio have also helped.
Jon Pertwee jumped at any excuse during the show to show his interest and skill in motor vehicles of any sort. Consequently, he was the only Doctor to drive motor vehicles (Bessie and the Whomobile) on a regular basis.
Early on in his run, Sylvester McCoy would show skills of his from his old vaudeville act, such as playing spoons. As the show got Darker and Edgier, and as the Doctor's character became more complex, such displays were discarded.
Averted on several occasions: for example, despite having Kylie Minogue appear in a Christmas episode with a song interlude ("Voyage of the Damned" and song being "The Stowaway"), she is never called upon to sing; Billie Piper similarly was allowed to do straight acting and not have to sing; and with a few exceptions, Catherine Tate was allowed to tone down her comedic acting and play Donna Noble straight.
The First Doctor started off as someone, who'd in a moment of desperation, tried to bash in a injured man's skull in, to escape the present danger. He was stopped by a Human who called him on this, even though he was someone the Doctor had belittled as beneath him until then. This might explain why all of his later companions are mostly Human, because they do stop him, when he goes too far. Russell T Davies and Steven Moffat's runs seem to embrace this interpretation.
Some companions also get their fair share. Notable examples from the revived series include Jack Harkness, Donna Noble and Rory Williams.
Check And Mate: While the Doctor is frequently bluffing or stalling, when he isn't, you're already doomed. Run!
"The Christmas Invasion" (Dec 2005), it gets chopped off in a swordfight above Earth. It is (at some point) picked up by Captain Jack.
Torchwood, Series 1 (2006-07), Jack has a mysterious hand in a container for the entire first series.
"End of Days" (Torchwood) /"Utopia" (Jan/June 2007), Jack hears the TARDIS and sees the hand respond (bang), grabs it, and joins the Doctor.
"The Sound of Drums" (June 2007), The Master uses the Doctor's DNA (from the hand) in order to age him to reflect how old he actually is (bang).
"Last of the Time Lords" (June 2007), The Doctor takes back his hand and leaves it in the TARDIS.
"Journey's End" (July 2008), After being shot by a Dalek, the Doctor sends his regeneration energy into the hand and continues his adventure. Unknown to him, the hand grows into a full clone of the Doctor and imbues Donna with the Doctor's mind, effectively creating three Doctors, two and a half years after the hand was first cut off. (BANG)
And the Eleventh Doctor more and more, especially in Series 6.
Christmas Episode: "The Feast of Steven", the first (and, until New Who, only) episode to air on Christmas Day, which had no continuity to the main serial The Daleks' Master Plan. In New Who, an annual series of specials, which between 2005 and 2009 doomed London (usually, but not always present day London) in some way. Aliens also threaten the Earth in 2011's episode, though it's not the primary plot.
The Fourth Doctor's coat would change depending on the "genre" of the particular serial. The yellow-brownish one was more for adventure, red for action, and grey for horror or mystery.
In the Fifth Doctor's first season, the boys are in yellow and the girls are in purple.
As the 7th Doctor was growing Darker and Edgier his coat is changed from a light whitish colour to dark brown.
Since the Ninth Doctor, the time vortex is blue if the TARDIS is going backwards in time, and red if it is going forward.
The Doctor's bowtie in his eleventh incarnation will be red if the episode takes place in the future, and blue if it's in the past.
The rejuvenated Daleks have different colored casings to identify their functions, but thus far we haven't seen enough of them for it to really make a difference. Black Daleks are usually called "Supreme" and have leadership positions. And when they were divided into two factions in Remembrance of the Daleks, they had different colors as well.
Even the main character's name has been subject to this: The original treatment, and early scripts and end titles, are not at all clear about the idea that the Doctor's name is not "Doctor Who". This wasn't firmly established until later.
The Daleks could almost have their own page for this. In the original encounter, the Daleks had been living in their underground city for only a few hundred years, waiting for the radiation from a nuclear war to fade, only to discover their mutated forms needed radiation to survive. Their self-created "travel machines" could only operate on powered metal surfacesnote This was later bypassed with a power-receiving antenna dish mounted on their backs, but even that was soon forgotten., and even in-story stuck to smooth surfaces, ramps, and elevators. They were cold and cruel, but by no means super-intelligent. They were defeated in the Doctor's first encounter, before they had a chance to ever leave their city. By the time the new series got into action, they had become computer-integrated, universe-conquering, flying battle machines.
The Cybermen didn't achieve their trademark appearance until the Second Doctor serial The Invasion, their fifth appearance, and only gained a weakness to gold dust in Revenge of the Cybermen. Silver Nemesis flanderized this into an extreme weakness to all forms of gold.
Conservation of Ninjutsu: In "Dalek", a single Dalek manages to wipe out an entire base full of trained elite soldiers (and is only defeated because it decides to destroy itself). More recent episodes have seen entire armies of the supposedly terrifying and insurmountable space-Nazis regularly thwarted by a combination of technobabble and genetic wizardry.
In-universe, it's strongly implied that the lone Dalek was moments from being thoroughly blasted by the Doctor, and vast armies of Daleks are treated as the end of the world rather than Mooks. In practice, trope is fully in effect, though this seems to be more a case of the Doctor being able to beat the Daleks each time they appear regardless of the numbers even though they are a tremendous in-universe threat. Pretty much every time the Doctor isn't present or isn't really invested in their enemy surviving ( aka when Daleks fight the Cybermen, both were enemies and if either survived the survivor would take over the world) the result is that the Daleks pretty much curb stomp their opponent with the only real exception being the Time Lords themselves who were still losing. The Cybermen lose easily, and let's not forget that it took minutes for them to subdue earth in spite of tremendous preparations specifically for this eventuality.
Rose: Five million Cybermen, no problem. One Doctor? Now you're scared."
Convenient Replacement Character: When a companion leaves, the Doctor usually picks up another one either in the same story, or the following one. Considering that he frequently meets one-shot characters who have the potential to be companions, it's not entirely unexpected that vacancies in the TARDIS are usually filled quickly.
The Time War makes history far more malleable and rewritable, and effectively wipes out the Time Lords, the Daleks and Gallifrey from history.
The above might have been true during the first few episodes of the revival series, but once the Daleks were reintroduced in the episode "Dalek", this has been completely ignored, with Gallifrey, the Time Lords and the Daleks continuing to have a presence (either directly - as in The End of Time and multiple Dalek episodes), or in dialogue. There is no indication at all of them being wiped from history.
The "crack in time" in the 2010 series, particularly after what happens in the season finale, also leaves room for history to have been rewritten, although so far it hasn't been explicitly called on.
Cowboy Bebop At His Computer: It's become generally accepted, by fans and production alike, that The Doctor's name is not "Doctor Who", but the media doesn't seem to know this. Even the end titles sometimes list the character as "Doctor Who". (That last is less egregious of an error in early episodes, when the name distinction wasn't firmly established yet.)
Creator Thumbprint: Eric Saward's scripts and tenure as script-editor tended to involve a lot of gritty, hard-edged mercenaries and space marines, to the point where it started to seem that he was more interested in writing those characters than the Doctor and his friends at times.
Creepy Changing Painting: The Weeping Angels are the statue version; by biological necessity they only move when you can't see them but then move very very fast.
Creepy Children Singing: A recurring motif during the second half of New Series 6, particularly the episodes "Night Terrors", "Closing Time", and "The Wedding of River Song", is creepy children singing a foreboding nursery rhyme with verses that change to suit each episode.
Crew Of One: Rarely has the Doctor had a companion who could fly the TARDIS, or do much of anything besides simple button-pushing. And on the occasion a companion or foe does pilot it, it's still just the one. Though in "Journey's End", the Doctor pointed out that a TARDIS is supposed to be piloted by six people at once, and the reason it has so much Explosive Instrumentation going on is because the Doctor isn't quite filling in perfectly for the other five - though the fact it runs at all is probably proof that he's just that good.
Crystal Spires and Togas: Gallifrey, usually, and several other alien examples. Gallifrey may be a subversion; the crystal spires and togas help hide the stagnation and decay of Time Lord culture: a sufficiently advanced alien society that has rested on its laurels for ten million years.
Reinette: Doctor? Doctor Who? It's more than just a secret, isn't it?
The Dark Side: Season 23 The Trial of a Time Lord has The Valeyard as the ultimate villain. He turns out to be an evil version of The Doctor from somewhere between his 12th and 13th incarnations, trying to steal The Doctor's remaining regenerations and become an independent person. The Master helps The Doctor defeat him without any prompting or reward beyond being able to live in a universe that doesn't contain The Valeyard.
Typically, a teenage companion will be played by someone much older. Billie Piper is far closer to her character's age than other actors who play teenagers. Averted by Matthew Waterhouse (18, when he started) and Sarah Sutton (19, when she started) in the classic series.
Karen Gillan and Arthur Darvill (who we first saw playing their characters after they'd left school) played secondary school versions of Amy and Rory when they were 23 and 29 in "Let's Kill Hitler".
Also inverted with these two, as by the end of their run their characters are in their late thirties, while it has only been a couple years in real-life time.
Dead Alternate Counterpart: Inverted in the alternate universe featured in series 2, where Rose's dad and Mickey's gran are both still alive. The Doctor and Rose don't have counterparts at all (apparently the Time Lords don't exist in this universe, and Rose's parents never had kids, though they do have a dog named Rose); Mickey's counterpart... well, he starts out alive...
The Master has died on-screen without regenerating no less than three times. It's never stopped him from coming back for more. The show doesn't even bother to explain why his possessed Trakenite body is alive again in "Mark of the Rani", after burning to death in Planet of Fire.
The Russell T Davies era saw "The end of the Daleks" no less than three separate times, and yet everyone's still surprised when more Daleks show up. They wised up after a bit—out of those three "ends of the Daleks," two happened in series 1. After that, they made a point of ensuring that at least one member of the Cult of Skaro survived each encounter, until Russell T Davies decided to go out with a bang and did them in again at the conclusion of series 4. Naturally, this meant Steven Moffat had to go and dig them up again, but he's been careful to keep them alive since.
Deconstruction: Since its reboot in 2005, the show has been gradually deconstructing itself. The Doctor is, as always, an eccentric man with a saviour complex whose mystique both entices and frightens people, and these traits have increasingly tended towards tragedy for him. It started with realistic problems finding their way into the story, like a companion's family assuming that she has died, and the emotional consequences that followed. And it got worse. Russell T Davies assaulted the idea of the Doctor himself in "Midnight", in which all of the Doctor's normal methods of controlling a situation backfire entirely, and he is almost killed as a result. Soon after, in "Journey's End", he is shown his "true colours" when his companions are prepared to destroy themselves and the Earth if need be to stop the Daleks' plan. Steven Moffat has since deconstructed the Doctor's Memetic Badass status - it's satisfying to watch the odd monster of the week wet themselves when they realize who you are, but it makes others do desperate, terrible things to get the edge on you.
Distressed Damsel and Distressed Dude: The Doctor, in varying degrees throughout all his incarnations; most if not all of the companions, whether male or female, at some time or another; assorted bystanders of both sexes. Seriously, having someone taken prisoner or menaced by the Monster of the Week is one of the standard plots.
Divine Chessboard: In whole-season spanning story arc The Key To Time there were the White and Black Guardians.
Driving Question: "Doctor who?" It's been asked an ungodly number of times, and as of series 6, it's the oldest question in the universe, hidden in plain sight, and must never ever be answered. "Silence must fall when the question is asked."
During the War: The Time War and World War II are only the obvious examples.
Early Installment Weirdness: The show was originally conceived of as a strictly children's program with a strong educational component. Several early episodes take place in a "real-world" historical setting, with the only "sci fi" element being how the characters got there. The Doctor is prone to making speeches about how things work (though only if it serves the plot; he only ever breaks the fourth wall once, to wish viewers a Merry Christmas in a 1965 episode). This has all but disappeared even a few seasons in, with it becoming a pure adventure show. The last purely historical storyline, The Highlanders, aired in 1966, though a one-off return to the format, Black Orchid, appeared in 1982.
To elaborate, the Doctor was originally intended to be a borderline antagonist that kept getting the heroes into trouble. Indeed, in the second episode he almost brains a caveman to death with a rock, only to be stopped by Ian at the last second. It was Ian who was intended to be the show's main protagonist, and his and Barbara's professions (teachers, the former science and the latter history) are clear indicators that it was supposed to be an Edutainment show. Susan was to represent the viewing audience.
Enforced Method Acting: According to Word Of God, Alex Kingston (River Song), Karen Gillan (Amy), and Arthur Darvill (Rory) all know certain things about their character that haven't been revealed. These details have been intentionally withheld from Matt Smith. So when he doesn't know something, he really doesn't know.
Epic Hail: Whether it's his old companions hailing the Doctor, the whole population of Earth hailing the Doctor, the Time Lords hailing the Master, or Rose Tyler hailing herself, the Russell T Davies era loves to put an Epic Hail in his season finales. Series 2 is the only one he missed.
Eternal Hero: As a hero who saves the world in different ages, on different planets, and in different incarnations, The Doctor is a certifiable hero of mythological scope.
Everyone Calls Him Barkeep: The Doctor. (Actually, this seems to be a pattern for renegade Time Lords — see also the Master, the Rani, the Monk, ...)
Everything's Better with Spinning: The Eleventh Doctor's second control room has a lot of neat spinny bits - lights around the room, rings above and below the console, and probably more.
Expanded Universe: A huge one. The rabbit hole can literally be said to be endless, because each spinoff has its own spinoff, which ties into other spinoffs... etc. There are at least two hundred original novels covering all eleven Doctors, hundreds upon hundreds of audio dramas made by Big Finish and other audio companies covering all possible Doctors, companions and villains, and decades worth of comics, Web Original content, two Alternate Continuitytheatrical films and even several theatrical productions. Several of the Doctor's EU companions who have become well-known among the fandom, and a few characters — such as Kate Stewart and Joan Redfern — have actually made their way into the TV series. See Doctor Who Expanded Universe for more.
Exposition Beam: The Doctor, Eleven in particular, has the interesting ability to mind-meld information into other people's heads without having to explain it verbally, or the target having to be conscious, or even alive at the time. In cases of emergency, where a lot of information has to be imparted very quickly, The Doctor will opt to HEADBUTT the person in question, rather than simply mind-meld with them!
Exposition Of Immortality: Doctor Who does this all the time. Whether it's a serial Big Bad being outed as an alien who's been on Earth for centuries or the Doctor himself's longevity via regeneration. Encounters with recurring enemies the Cybermen and the Daleks frequently went hand in hand with a montage of past episodes or declamations by either side about their past encounters. It's a little confusing due to time travel being involved, but the Doctor definitely qualifies, as he himself keeps on aging in between encounters, and there's often several hundred years or both his and his enemies time between each confrontation.
Females Are More Innocent: The original show ran for a quarter century and had a large number of villains yet in that time period only about 10 were women, and only one or two of them appeared in the show's first 15 seasons.
Feudal Future: Various planets the Doctor's landed on, from time to time.
Fingertip Drug Analysis: The Doctor's Bizarre Alien Biology lets him taste things safely...including human blood. "Planet of the Dead" goes further; he can sense traces of cities and mountains in the ravaged world of San Helios' sand. In "The Eleventh Hour", he can tell the exact age of a shed by licking it. In "Day of the Moon", he can tell where the TARDIS-blue envelopes from the previous episode were made from licking.
Sontarans versus the Rutans. It's been going on for 50,000 years as of "The Poison Sky", and is still going at least 10,000 years after that in The Sontaran Experiment with no end in sight. Both sides are perfectly fine with this.
The Kaled-Thal war went on long enough to turn Skaro into a wasteland and reduce both sides to archaic weaponry - then came the Daleks.
For the Funnyz: Leave it to the Doctor to make quips and resort to measures with an amusing/ironic edge.
Free-Love Future: By the 51st century, the home time period of Captain Jack and the adopted time period of River Song, humans will do it with anything.
Freeze Frame Bonus: In the new series credits starting with the addition of companion Clara in the back half of season seven, the Doctor's face shows up briefly. It's actually all 11 doctors in an instant.
Gambit Pileup: Common in stories involving the Master and/or the Daleks. Over the decades, the entire history of Earth has started to look like one - it formed in the first place because the Racnoss needed somewhere to hide for a few millenia, and a horde of different outsiders (Scaroth, the Daemons, the Exxilons, the Osirians, the Silence) have attempted to guide human development for their own ends.
Gender Bender: It was hinted that regeneration can do this in The End of Time, and confirmed in "The Doctor's Wife", in which the Doctor mentioned that this has happened to another Time Lord, the Corsair, on several occasions.
Genre Roulette: borderline Genre Busting at times. It did so even more in the era of William Hartnell, who played the First Doctor, before the series had quite settled into its format. As showrunner Steven Moffat put it: "Sometimes it's comedy, sometimes thriller, sometimes horror, sometimes children's stories, the silliest stories you've ever seen. Sometimes it's all that in the same episode." On the Nerdist podcast, Matt Smith praised the format for not being bound by "logic, time, space, or genre."
Genre Savvy: All of London. The Doctor finds her streets vacant on Christmas Eve one year not because of something sinister but because there have been two previous sinister Christmas plots in London on consecutive years and the citizens weren't about to try their luck a third time.
A Good Name for a Rock Band: Many things in the Whoniverse sound like really good band names. The show itself has spawned its own genre of music: Trock or TRock (Time Lord Rock). Some of the more famous bands who take Doctor Who references as a name are surely Chameleon Circuit and The Cult of Skaro, but a whole list of Trock bands can be found here.
Great Offscreen War / Götterdämmerung: The Time War, taking place at a scale so epic it broke space and time. Ten million Dalek ships versus the full power of the Time Lords, with things with names like "the Nightmare Child" and "the Could-Have Been King" appearing out of the crossfire. It ended with the Doctor setting fire to both worlds, and now it's just him, the Master (sometimes) and whatever Dalek remnants are still out there.
Guns Are Worthless: Especially egregious during the UNIT era. Guns are utterly, hilariously, ludicrously useless against almost anything even slightly non-human UNIT encounters.
Played straight in "The Sontaran Stratagem" as the titular bad guys have a way of expanding the copper in bullets and causing guns to jam. Quickly and awesomely averted in "The Poison Sky" when UNIT switches to bullets without copper.
Heroic BSOD: The Tenth Doctor is seen to do this on a couple of occasions (most notably "The Stolen Earth") when his insane ingenuity has failed him and he can't think of anything to do—he simply stands there, motionless, his face blank and fixed. It's fairly creepy, in fact.
Heroic Sacrifice: Extremely, extremely common. At least half the Doctor's Regenerations have been this.
Quite soundly averted in Mawdryn Undead, where the Doctor attempts to sacrifice all his remaining regenerations to allow the tormented scientists to finally die. The Brigadier happens to short out a time differential at just the right microsecond to supply the needed energy.
High Tech Hexagons: The insides of the TARDIS walls often have a hexagonal pattern on them, and the TARDIS console is hexagonal.
Although one toy manufacturer got it wrong and marketed a pentagonal version.
In the "pilot episode" version note Episode 1 was significantly remounted between this iteration and the one broadcast. The two episodes appear on the DVD set separately. of An Unearthly Child Susan Foreman is demonstrated to be unearthly by...making an inkblot inside a hexagon.
Historical In-Joke: Hello, it's a show with time travel... In fact, this trope is so common the jokes frequently mutate into running gags, like the one about the "Virgin" Queen.
Home Sweet Home: The Doctor never evinces this, but it causes some companions to leave.
Hopeless War: The Last Great Time War ended up as one of these. Countless soldiers were being killed and revived in time loops, the crossfire was spawning cosmic horrors by the truckload, and the Time Lords were so desperate to survive they brought back Rassilon, whose victory plan amounted to destroying the rest of the universe so they could become beings of pure thought. The Doctor had to wipe out both sides rather than let either of them "win".
Excepting some physiological differences that aren't readily apparent (the two hearts thing, etc.), Gallifreyans are indistinguishable from humans, at least on the outside. The companions Adric, Nyssa, Turlough and Astrid are neither humans nor Time Lords, but are physically indistiguishable from either.
Countless other alien species such as the Kaleds, Thals, Kinda, and Apalapucians are physically identical to humans.
The series handwaves this in bits by saying other planets have humans on them as well, before people from Earth boldly goes.
It has been theorised that this trope has happened with the Doctor to some extent, although given that his personality changes with each regeneration it's a bit hard to pin down exactly how much humanity has rubbed off on him.
It has happened to individual Daleks on more than one occasion.
Human Outside, Alien Inside: Time Lords have two hearts with a redundant circulatory system, a low body temperature, a respiratory bypass system, an ability to regenerate from death twelve times, a lifespan of potentially hundreds of years per body, and a complete additional sensorium tuned to temporal events.
Humans Are Bastards: A number of stories have shown future humans abusing (or enslaving) other humans or aliens. As Gwen says in Torchwood: Children of Earth, "Sometimes the Doctor must look at this planet and turn away in shame."
Humans Are Morons: In the first season, there comes a moment when the Doctor wonders why he likes humans so much, seeing as we have "such limited brains." His later incarnations seem to have pretty much the same thought. Indeed, when he's not going on about the marvelousness of humanity (a particular feature of the Tenth Doctor), he's ranting about how stupid/blind/ignorant they are. Sometimes in the same episode.
Iconic Item: The sonic screwdriver, especially post-2005, where it commonly appears in publicity pictures.
Iconic Outfit: Every Doctor's uniform is iconic of that incarnation. Special mentions go to the Fourth Doctor's scarf and fedora, and the Eleventh Doctor's bowtie (which was inspired by the Second's).
Identical Stranger: At least two Doctors, as well as his companions Romana and Nyssa, have met identical strangers. It's a big universe, and there's only so many faces to go around. (The Doctors' doppelgangers were both villains; Romana's and Nyssa's were distressed damsels, which is not necessarily an improvement.)
I Did What I Had to Do: The end of the Time War: it was established during the Ninth Doctor's run that the Doctor had ended the War by killing all the Daleks and the Time Lords, he revealed in The End of Time the reason he'd done so was that after years of war, the Time Lords had become just as vicious and brutal as the Daleks, and would have been an even worse threat to the universe.
Idiosyncratic Episode Naming: Many Dalek-centric serials (mostly in the classic series, but the revival has occasionally made nods to it) have been titled "[blank] of the Daleks". The Nathan-Turner era stories used the theme of R-words like "Resurrection", "Revelation" and "Remembrance".
Particularly in the new series, the first appearance of a Dalek in a story is almost always accompanied by the all-purpose Catch Phrase "EXTERMINATE!" Variations include "HALT! STAY WHERE YOU ARE! YOU ARE A PRISONER OF THE DALEKS!"
In all his appearances, The Master tends to announce himself with "I am the Master, and you will obey me."
Indy Ploy: All of the Doctors except the Seventh tended to use this technique.
Intro Only Point Of View: Both the old and the new series begin with contemporary Earth humans puzzling out the mysterious happenings triggered by the Doctor. They shift to more minor points of view. Many episodes start with the point of view of characters in the situation where the TARDIS will arrive; some do not survive the opening, and others become much less important POVs once the Doctor arrives.
Kill 'em All: Usually the Doctor and his companions survive, but they have been known to depart from places where all known characters died, or there might be minor survivors in the odd corners.
Leaning on the Fourth Wall: At the end of series 6, we hear of the "oldest question in the universe, hidden in plain sight". It is actually only hidden in plain sight for the viewers of the show, since it's in the title.
Loads and Loads of Characters: The core cast is the Doctor and between one and three companions. Supporting and recurring cast is...somewhat higher, especially when taking into account all the alien races in 30 seasons of show.
Long Runner Cast Turnover: The series of course invented The Nth Doctor in order to survive 40-plus years but also rotates though a long list of the Doctor's companions; the show's time travel premise makes it fairly easy to write old companions out and new ones in.
Loss of Identity: Addressed at each regeneration and when he became human in "Human Nature"/"The Family of Blood".
Male Gaze: The Doctor's companions have consistently been attractive young women, with some dressed in, shall we say, less than practical clothing. Ya'know, considering they're running around all of time and space, often with things with sharp teeth just a step behind.
This  article covers different aspects of the issue quite well, (lovingly) using Amy Pond as an example.
Meaningful Background Event: Used a lot in the new series. It will usually be something relating to the finale of that series. Series 1 had the words Bad Wolf scribbled everywhere; Series 2 had everyone and their mother mentioning Torchwood; Series 3 had posters saying Vote Saxon; Series 4 featured various TV screens briefly showing Rose screaming and so on.
Mechanistic Alien Culture: The Daleks and the Cybermen are both technically cyborg races that follow this trope; particularly the Cybermen. They both have robotic voices and overlap with Transhuman Aliens. They are both obsessed with machine-like efficiency, but the Daleks are dedicated to exterminating other species (especially humans and Time Lords), whereas the Cybermen are The Assimilator.
Missing Mom: Clara Oswin Oswald's mother is shown to have died early on in her life.
Monster of the Week: Almost every episode of the series features the Doctor facing a different alien, mutant, or robotic creature.
Revisited with Amy between The Doctor and Rory. The Muggle wins!
Mundanger: Although the earliest episodes alternated between science-fictional and purely historic episodes (the series started out as an educational show, you see), it soon evolved into a purely sci-fi show. The only post-'60s episode to feature a completely mundane threat was the Fifth Doctor story Black Orchid.
Steven Taylor, one of the First Doctor's Companions, would often call him "Doc". The Doctor would demand that Steven call him by his proper name.
Negate Your Own Sacrifice: Standard modus operandi for the Doctor for obvious reasons, which frequently results in regenerations. Occasionally he gets aid from himself from another point in the timeline to bail him out.
All attempts at Heroic Sacrifice by Jack Harkness end this way, him being unable to die and all.
Nephewism: Prior to the new series, as companions had aunts or uncles as had parents. Dodo Chaplet was living with her great-aunt when she met the Doctor, and Tegan Jovanka was living with her Aunt Vanessa. Sarah Jane Smith was raised by her Aunt Lavinia. Jo Grant's doting uncle gets mentioned several times. In the new series, Amy Pond was raised by her aunt prior to the series 5 finale.
No Equal Opportunity Time Travel: Somewhat Depending on the Writer. Martha, as a black companion, gets away in odd settings with an appropriate alibi and some smooth talking from the Doctor. "Human Nature"/"The Family of Blood" is an exception: the Doctor can't help her and she encounters resistance because of her race, gender and "servant" status.
No Export for You: Americans had to wait until 1972 before finally seeing an episode on their screens. Nearly happened again in 2005 when US broadcasters initially refused to buy the new series, reportedly because it was "too British".
From about 2006 to 2010 it was commonplace for online content produced related to the series to not be viewable outside the UK (behind the scenes videos, prequels, games etc). This has improved in the last couple of years, but there is still extensive content that cannot be viewed outside the UK (at least until someone posts it to YouTube...).
Although some original content eventually makes its way onto DVDs that get released in North America, there have been some notable exceptions, such as the "TARDI Sodes", a series of prequels released online in the UK only and never included on DVD, and Captain Jack's Monster Files, a web series featuring John Barrowman that remains web-only in the UK.
Initially applied to a series of computer games produced for online distribution beginning in 2010. Eventually were made available via a retailer with North American release rights, except for the Mac version, even though it was available for Mac in the UK.
In a minor reversal, only viewers in North America and Australia viewed a unique pre-credits sequence narrated by Karen Gillan that was added to non-UK broadcasts of Season 6 in order to introduce new viewers to the show. This opening never made it onto the DVD releases in the UK or anywhere else.
No Periods, Period: Despite constantly dragging young women along on his travels, the Doctor never has to deal with this particular obstacle. Wibbly-wobbly, timey-wimey.
One Myth To Rule Them All: A lot of myths and legends turn out to be aliens, usually with the Doctor saving the day. Or, as River Song put it:
"I hate good wizards in fairy tales. They all turn out to be him."
One Steve Limit: Not rigidly adhered to; in a series this long-running, some names inevitably repeat. The number of unrelated characters with the last names "Smith" note Mickey Smith, Sarah Jane Smith, and the Doctor's alias John Smith, or "Jones" note Ianto Jones, Martha Jones, Harriet Jones, Clifford Jones seems to be on the excessive side. Russell T Davies is from Swansea, and he liked his Welsh family names.
Opening Narration: Done only during Amy Pond's run as the Companion to Matt Smith's Doctor, giving a jaded, yet enchanted take of The Eleventh Hour.
Orange Blue Contrast: Present in all episodes of the 2005 revival (except maybe Series One, which prefers green and purple as the contrasting colours), especially in Moffat's episodes, most of which employ a heavy dose of blue. This also contrasts with Russell T Davies, whose episodes tend to favour orange.
Very early on in the classic series run the "rules" of time travel transitioned from "you can't change history... not one line" in the season one story The Aztecs to manipulation of history being the plot of the season two story The Time Meddler.
Certain moments in time are "fixed" and "part of events" so they cannot be changed, others can be.
If a paradox is created monsterous Clock Roaches show up to "cauterize" the wound as seen in "Father's Day". It could only apply to certain types of paradox. Or sometimes there is a big explosion as seen in Mawdryn Undead.
Paradoxes may be subverted by using a 'Paradox Machine', basically a retrofitted TARDIS as per "The Sound of Drums" and "Last of the Time Lords".
Our Vampires Are Different: State of Decay, The Curse of Fenric, "Smith and Jones" and "The Vampires of Venice" each featured different variations on the standard bloodsuckers.
Our Werewolves Are Different: The series has also had several different versions of the werewolf. The creature in "Tooth and Claw" is identical to your standard pop culture werewolf...except that, like almost all monsters on the show, it's actually an alien. The werewolf in "The Greatest Show in the Galaxy" and the werewolf-like creature in "Planet of Evil" diverge more from the template; of course, they're both aliens as well.
Percussive Maintenance: Thumping things often gets them working again... including the TARDIS. In the revived series, the Doctor has a mallet hanging from the console on a piece of twine for just that.
Phony Newscast: Common, often using real BBC newsreaders, in the present-day episodes in the Davies era of the show. Used from time to time in the old series, too (most notably in the Jon Pertwee era), but as of series 7, only used once in the Moffat era.
Primal Fear: How the show is so scary, especially in the Moffat era and Davies era stories written by Moffat.
Red Herring: The new series is especially fond of throwing these out for viewers trying to figure out the its various ongoing mysteries. The Arc Words for the 2009 specials, for example ("he will knock four times") were given at least four or five false explanations before we found out the truth (and if you add fan speculation, go ahead and multiply that number by a couple of thousand.)
Ret Gone: What happens to those who fall through the cracks in the universe. And Steven Moffat has reportedly said that, as a result of the cracks (and what was necessary to fix them), a number of major events of the preceding series (most notably, the several alien invasions and other incidents that attracted widespread public notice) have been erased from history, leaving an earth that is once again (almost) entirely ignorant of the existence of alien beings. What this means for the past Companions whose travels with the Doctor hinged on those events is anybody's guess.
However much of this was undone when the Doctor rebooted the universe in The Big Bang, which is stated on screen as restoring everyone's memories.
As of Asylum Of The Daleks, the Doctor has erased himself from every database ever, so nobody remembers him.
Reverse Polarity: Multiple examples, including "Reverse the polarity of the neutron flow" and the sonic screwdriver.
Ripple-Effect-Proof Memory: Part and parcel of being a time traveller. One's perception of history is altered, forever, allowing you to remember what the world was like before history was changed via Retcon and Ret Gone. However, it isn't retroactive.
Rule of Perception: Duck into the corner or behind that pillar, crouch behind the lab bench, etc. etc. etc. Make sure the audience can see you. Even by Rule of Perception standards, it's amazing how conspicious the characters can be without being found by the search party.
The TARDIS' broken chameleon circuit and wonky steering.
"It's bigger on the inside."
In the Christmas specials, the fact that it never really snows at Christmas beyond the 19th century.
The tendency of new companions to attempt to speak in foreign accents (and, of course, failing spectacularly) with the Doctor just wincing at them saying, "Don't...don't do that."
And the oldest running gag in the whole series: Occasionally the Doctor will introduce himself as "Just the Doctor," and someone else in the scene will ask, puzzled, "Doctor who?"
"Harriet Jones, Prime Minister." "Yes, I/we know who you are."
During the Eleventh Doctor era, it's become a sort of in-universe gag that Rory will die again and again. At one point, even The Silence mention his tendency to die a lot.
Noodle Incidents were occasional in the series, until the Steven Moffat era, where we get as many as three an episode.
In the new series, dramatically running.
As lampshaded in "The Doctor's Wife", the instructions written on the TARDIS' door quite clearly read "Pull to Open". *
Since real Police Boxes did open outwards, not inwards.
Despite this, in all the time they've travelled together (and indeed, most of the series), The Doctor has stubbornly refused to do anything other than Push the doors open!
San Dimas Time: Despite being a show about time travel, almost all recurring characters always seem to remember their last encounter from the same perspective, and the Doctor's idea of "present day" always agrees with the audience's. It is taken for granted that Time Lords meet each other in sequence, due to a presumptive "Gallifrey Standard Time". Of course, it's also easier to run a recurring character if you can refer back to the previous encounter. (The only exceptions to this in the old series were multi-Doctor anniversary stories, and Trial of a Time Lord. In the revived series, Have We Met Yet? incidents have become much more common.)
Sci-Fi Writers Have No Sense of Scale: A temporal example. Some episodes take place hundreds of thousands, millions, billions, and even in one case trillions of years in the future, yet the relative progression of human existence (technology, evolution, society, etc) could plausibly be from the next few hundred years or so. It is likely that human civilization would have evolved to the point of incomprehensibility to a 21st century observer. Lampshaded a couple of times, with the Doctor explaining that in the future humanity will go through phases of being more or less similar to the present, and sometimes deliberately recreating the 21st century out of nostalgia.
Screwed by the Network: Happened quite a few times in the 80s, which ultimately put the show on a year and a half hiatus and was the reason Colin Baker was fired.
Sealed Evil in a Can: Around half a dozen cases of Evil deliberately Sealed in a Can. At least as many slumbering alien menaces that just happened to crash-land/get trapped centuries ago and go into hibernation until foolishly awoken, which don't technically fit the description but serve a very similar plot purpose.
Inverted with The Pandorica. The alliance believes that they are doing this and saving the world by locking the Doctor away, but they are in fact dooming the world to destruction, as he is the only one who can stop the TARDIS from exploding.
The Dalek episodes from "Army of Ghosts" to "Journey's End" directly follow on from one another. The surviving Daleks from "Victory of the Daleks" came from the Dalek invasion of "The Stolen Earth". A similar use happens with Cybus Cyberman stories from "Rise of the Cybermen" to "The Next Doctor".
Satellite Five (and all the mess the Doctor's involvement caused) reappears in the series 1 finale, after an assumed one-off encounter with the Mighty Jagrafess in "The Long Game".
While Torchwood was a recurring Arc Word in Series 2 (with the Doctor meeting a group identifying as the Torchwood Archive in "The Impossible Planet" and "The Satan Pit"), the Doctor's only direct encounters were at the founding in "Tooth and Claw" and his capture by (and the subsequent slaughtering of the team of by the real villains) Torchwood London in "Army of Ghosts".
Series 4, 5 and 6 have a recurring string of non-consecutive River Song episodes ("Silence in the Library"/"Forest of the Dead"; "The Time of Angels"/"Flesh and Stone", "The Pandorica Opens"/"The Big Bang", "The Impossible Astronaut"/"Day of the Moon", "Good Man Goes To War"/"Let's Kill Hitler"..., though from her point of view, they're (mainly) prequel episodes.
The End of Time picks up from The Master's death at the end of "Last of the Time Lords".
The cancellation of the classic series in 1989 was originally described as a "hiatus" by the BBC. And it was, of a sort. It just lasted considerably longer than the earlier one: at least seven years (until the 1996 TV movie) and up to sixteen years (until the series resumes on a regular basis with Rose in 2005).
A semi-example in 2009 when there wasn't a proper series, just a bunch of specials throughout the year.
Shakespeare In Fiction: A major role in the plot of "The Shakespeare Code", and a brief cameo in The Chase. In other episodes, the Doctor alludes to offscreen meetings with him.
Share Phrase: Nearly every person who stepped into the TARDIS for the first time said, "It's bigger on the inside [than on the outside]", to the point where on one occasion the Tenth Doctor silently mouthed the phrase along with them, and the Eleventh later became surprised when those that enter it don't say it.
Skeleton Key: The Doctor's sonic screwdriver often acts as one of these.
Space Whale Aesop: Several here and there. For example, "Use clean, renewable fuels, because sometimes the thing you're using for fuel is sentient, angry and capable of possessing you." — which has been the aesop of more than one episode.
Notably there was an episode with a space whale aesop involving an actual space whale.
Superman Stays Out of Gotham: The Doctor has appeared in The Sarah Jane Adventures twice, but there really isn't much justification for why he leaves saving the world in all the other episodes up to a middle-aged woman and a bunch of kids. Torchwood sort-of lampshaded it when Gwen wonders if the Doctor is looking down on the Earth in shame.
Talkative Loon: One of the possible temporary side effects of a Time Lord's regeneration, as evidenced by the Fourth, Seventh, Tenth, and Eleventh Doctors, as well as the Master's only on-screen regeneration.
Take That/Self-Deprecation: Any time two or more incarnations of the Doctor meet, it's a safe bet they'll have something snarky to say about one another's looks or attitude ("A dandy and a clown?", as the First memorably greets the Third and Second, to their faces). Counts as a Take That between the actors andSelf-Deprecation for the character.
When Ten met Five, Ten kept snarking about Five's celery lapel while Five kept treating Ten like a lunatic.
When the Ganger!Doctor's personality briefly reverted through previous incarnations, he channeled the Tenth's voice for a few seconds, before quickly reasserting control with "Nooo! Let it go! We've MOVED ON!"
The The Title: Prevalent throughout the show's run, though markedly less so in the Fifth Doctor's era in particular. During the Second Doctor era, only "Fury from the Deep" did not start with "The", and even its working title was "The Colony of Devils".
Theme Naming: After series 4, companions had a tendency to be named after bodies of water. Jackson Lake, River Song (a.k.a. Melody Pond), Amy Pond, Adelaide Brooke ...
This Banana Is Armed: "Pssh, what could a screwdriver do?" is a pretty common reaction at first. But did we mention it's sonic?
Threesome Subtext: Pretty much any time the Doctor has two companions (especially in the new series), this comes up. Favorites include Two-Jamie-Zoe (Sleep Cute), Nine-Rose-Jack (plenty of flirting, all three pairings kiss in their last episode) and Eleven-Amy-Rory (the Doctor resolves the love triangle issue by matchmaking them and whisking them both off after their wedding - then he marries their daughter, which complicates things...)
Time Paradox: Happens often. So much so that in the newer series, there exists a thing called a Paradox Machine which prevents time from healing itself in the face of a temporal contradiction. So, for example, you could kill your own grandfather and the Paradox Machine would ensure that nothing happened to you.
Uncommon Time: The Eleventh Doctor's theme is almost entirely in 7/4 (7+4=11).
Uniqueness Decay: In the early years, we knew almost nothing about the Doctor's people. It was six years before we learned the name "Time Lords". From the Tom Baker serial The Deadly Assassin onwards, we began to learn more and actually visited Gallifrey. Over the next decade or so, more stories featuring the corrupt, self-interested and machiavellian Time Lords were made, to the point where many fans complained that too much was being explained and the mystery had gone. One of the objectives of the so-called "Cartmell Master-plan" in the late 1980s was to Retcon some of this and reintroduce the mystery.
Unreliable Canon: especially before the New Series, the show can be best summarized as existing in a state of constant flux. Not only did you have the TV show introduce a new concept that gets changed to outright retcon after a few years, convoluted timelines for UNIT and the Daleks. This isn't helped by the fact that the New Series sometimes makes references to the books and that the New Series itself doesn't follow certain things from the Classic Who (the aforementioned Daleks' timeline for example). And let's not even get into the Expanded Universe...
We still haven't explicitly been told who was who was behind the plan to destroy the universe by blowing up the TARDIS in the series 5 finale. The episode sets this up as a plot point for the next series, which still doesn't explicitly tell us who was behind it. It's implied that the Silence were behind it, but that just raises even more questions, such as why, or how. Given that the Silence are a religion devoted to assassinating the Doctor to prevent him from answering a question he is apparently predestined to answer, and which they believe would cause the end of the universe if answered there, it's entirely unclear why they would attempt to create a rift in space and time that would destroy the universe, unless they were somehow unaware that creating said rift would lead to the end of the universe. Given how Crazy-Prepared they have been throughout the series, this seems unlikely.
The episode "The Lodger" also does not reveal who was behind the attempt to build a TARDIS; however, the two-part opener of the sixth series heavily implies that the Silence were behind that, since we see them occupying an identical room.
The Name of the Doctor doesn't actually reveal his birthname.
The Doctor, despite going back and forth through time and space, rarely (although there are exceptions) seems to have his outfit questioned, though his companion(s) might.
Then there's the blue box that pops up in the weirdest places and very few people think to question... even when they do notice it, they're more likely to write it off as someone else's problem rather than investigate thoroughly. The TARDIS was explained by the Doctor as generating a perception filter, making any normal being subconsciously want to look away from the blue box. This explanation coming from his companion's inquiry as the Doctor had fashioned a miniature version of the field to avoid detection by police or rival Time Lords, so it not only made sense from a story view-point, it finally answered the question of "Why then doesn't anyone notice a giant blue police box just sitting there!?" Especially when the Doctor has gone to places like Ancient Rome or 19th Century England.
Subverted in "The Fires of Pompeii" when the Doctor and Donna land in Pompeii on Volcano Day, leave the TARDIS for a couple of minutes...and return to discover that it has been carted off as art.
Also lampshaded in the Eleventh Doctor adventure "Meanwhile in the TARDIS," when The Doctor explains to Amy that the TARDIS' chameleon circuit (which allows it to change its appearance) DOES work, but that the TARDIS scans the local surroundings, determines the best camoflauge, and still configures itself as a 1960s Police Box.
Used Future: Despite being far ahead of most technology even approaching the end of the universe, the TARDIS is seriously broken and worn out by Gallifreyan standards. The Time Lords were also phasing out that particular TARDIS model for being outdated when the Doctor nicked it.
Video Inside, Film Outside: Throughout most of the classic series, though Spearhead from Space was entirely in film.note It was, in a sense, entirely "outside", as even the indoor scenes were filmed on location (mostly in a BBC training centre), due the studio being closed down by industrial action. Some episodes shot studio segments on film to make them "feel" outside. The Restoration Team website details many of the technical challenges associated with remastering the film sequences.
Viral Marketing: There have been, at various times, an actual Cybus Industries website, a conspiracy site, and another telling you to vote for Mr. Saxon.
World of Silence: The rationalization for creating the Cybermen (particularly the parallel-Earth breed) and the Daleks. The Cybermen think they're eliminating pain and strife by making everyone emotionless and identical, while Davros created the Daleks because he believed peace between species was impossible. The order actually called "the Silence" are a lot less clear in their goals.
The X of Y: By far the single worst abuser of this trope, guilty of it no less than 121 times. Having trouble coming up with an episode title? Try mixing and matching these ones that already exist.
You Keep Using That Word: Dimensionally transcendental. Transcendental means "Of or relating to a spiritual or nonphysical realm". Quite possibly the word they were looking for is transcendent, "going beyond ordinary limits", e.g. it goes beyond the physical dimensions of the blue box on the outside.
You Look Familiar: Such a long-running series inevitably has numerous instances of actors appearing multiple times in different roles, sometimes decades apart. This includes seven companions — and one Doctor — played by actors who had previously appeared in guest roles, and three cases of guest roles going to actors who had previously played companions.
Zeerust: Both intentional (the TARDIS's controls look rather clunky, possibly partly because of its dodgy condition) and unintentional. Basically every story set in space or and/or the future from the first eleven years of the series by now looks absurdly out-of-date, though the bell-bottomed space uniforms of the 70s now look oddly fashion-forward. And while the late 1960s stories makes some gestures toward internationalism, they almost always show show a preponderance of men in technical or scientific roles.