->''"For a bunch of guys on a mission to save the world, you sure do love your detours.''"
-->-- '''Gig''', ''SoulNomadAndTheWorldEaters''

A single plot element that was once a minor part of TheVerse swells in importance as the series progresses, growing more in focus and elaboration to the point that it becomes the focus of major arcs and plot development. As a result, the Plot Tumor's tentacles get wrapped around other elements of the work, either via {{retcon}}s that connect the Plot Tumor to things it wasn't originally conceived for or just by crowding out other elements in the story.

Typically, this occurs when the creative reins pass on from one writer to the next, as writers forget the original quirks of the element or their creative juices enable them to actualize the untapped story potential of taking a small but notable aspect and expanding on it. However, it's most visible in DerivativeWorks, where a single element that was important in the source material's success becomes the major focus of the adaptation, especially if late-to-the-party fans or non-fans of the original don't realize the Plot Tumor was once a small part of the original storyline when the work was first being published and it can seem almost unnatural for it not to be part of the status quo of a derivative work.

Keep in mind, however, that TropesAreNotBad and [[FanWank fandoms expound on minor details]] just as much as canonical writers; sometimes with the latter [[AscendedFanon ending up into the former]].

Compare: AdaptationDecay, {{Flanderization}}, RomanticPlotTumor, NeverLiveItDown, and MotiveDecay. A MalignantPlotTumor is the single-plot counterpart, where a minor plot at the beginning crowds out the other plots at the climax. Contrast AdaptationInducedPlotHole.
----
!!Examples:

[[foldercontrol]]

[[folder: Anime and Manga ]]
* ''Manga/YuGiOh'': Possibly one of the finest examples in fiction. The original manga began focused on many types of games and had Duel Monsters (Magic & Wizards) as an on-and-off card game, it was almost a SliceOfLife story where the heroes played all sorts of games with each other or villains used game-themed schemes. That's why the franchise's title translates to "King of Games", as in all games, not just Duel Monsters. But fans kept asking if there were real versions of those cards available and if the game would be revisited. Thus it became the focus of a few story arcs in the manga, became the ''central'' [[MerchandiseDriven focus]] of the [[Anime/YuGiOh anime]] loosely based on said manga, and it snowballed until the entire franchise centers around it.
** The mystical aspects of the game has also become this; in the original manga, there was a Shadow Game from Ancient Egypt involving the Millennium Items pulling monster spirits out of stone slabs (which were spirits that came directly from people) that the game's creator was tricked into bringing back as a card game, which is otherwise normal until turned into a Shadow Game by the present-day holders of the Millennium Items. This method of "bringing the game to life" happens to pretty much every type of game in the manga, not just Duel Monsters. But in the second series anime adaptation and it's subsequent spin-offs, it eventually evolved into an entire ''series'' of {{Alternate Dimension}}s full of monsters, and eventually into a power that's OlderThanTheyThink and is part of ''the Earth itself'' to the point where the cardboard alone is the source of supernatural occurrences.
* KiAttacks and powering up in ''Manga/DragonBall Z''. At the beginning of the original Dragonball, there were no KiAttacks, and the first of them, the [[KamehameHadoken Kamehameha]], didn't appear until the middle of the first arc. Even then, it was sort of the trump card, came at a high cost, and wasn't played terribly often. As the series progressed, though, the [=KameHameHa=] became a more standard attack, and Ki Attacks became more and more prominent. Then, DBZ came along and it became the main premise behind practically everything the fighters did. They could fly, teleport, power up, etc., all based on Ki manipulation. KiAttacks eventually led to BeamSpam, and the ability to power up that was introduced early in DBZ became the method by which nearly every BigBad but the last one was defeated, by digging just a little deeper and becoming just a bit more powerful.
** Flight is also this, when introduced it was a technique that only a few characters had, later almost every single character who fights had this ability.
** It's also interesting to note that ''{{Dragonball}}'' started as a homage to JourneyToTheWest, then it got a little martial-arts focused and drifted so far, you almost forgot the original ''purpose'' of the story was to find the Dragon Balls. By two-thirds of the series gone past, the balls were so easy to recollect again by the good guys, and the bad guys were no longer focused on getting wishes from them and settled for the destruction of the world and the rest of the cosmos.
*** By the first arc of DBZ, the Dragon Balls are even assembled off camera.
* The Uchiha clan in ''{{Naruto}}'', and Sasuke in particular, have done nothing but become more prominent in the story as time passes. It's gotten to the point where the Uchiha clan is responsible for the entire plot of the manga. Sasuke started out as merely TheRival to Naruto (though he clearly had greater story importance than other such rivals due to also being Naruto's teammate) who wanted to avenge his clan, but as the Uchiha presence expanded, so has his. He's arguably had more face time in the manga than the actual protagonist (he hasn't, though he's had more than every other character despite being largely absent for the first three arcs of Part 2), and in the arc that's shaping up to be the climax of the series, it's mainly Uchihas who accomplish anything of importance [[spoiler: since two of them are the main villains, and a third single-handedly negates the mass revival technique that nobody else could stop]]. [[BrokenBase It's a sore spot between fans whether this is a good thing, a bad thing, or something in between.]]
** The first hokage possessed a rare ability to control wood, something done by combining water and earth elemental chakra. In the beginning, this was just one of ten possible chakra combinations, with no reason to believe it was any more special than lava chakra or ice chakra. As the series has progressed, however, the importance of the first hokage's cells, which possessed this ability has grown, to the point where the amount of characters that possess them rivals the amount that possess the sharingan.
* Yukito Kishiro derailed ''BattleAngelAlita: Last Order'' for a two-volume gothic vampire story which acted as last-minute background for "Fata Morgana" (a nanotech super-program). It would appear the Fata Morgana became such a Plot Tumor due to Kishiro's understandable reluctance to pull a DeusExMachina on his readers, but it's still not the most elegant arc of the series).
* Quess' subplot in ''CharsCounterattack'' which ultimately hurts the film as a whole. Quess' story contributes little to a movie that was supposed to be about resolving the Amuro vs Char conflict, and takes away screentime from other, more interesting characters.
[[/folder]]

[[folder: Comic Books ]]
* In the SilverAge ''Franchise/{{Superman}}'' comics, Kryptonite went from a simple AchillesHeel to a rainbow of GreenRocks that could do anything, and was present in ludicrous quantities. This was toned down PostCrisis but has been brought back in recently. {{Lampshaded}} in an issue of SupermanBatman where Superman is almost accidentally killed because it was cheaper for a film company to use real kryptonite rather than to make a prop.
* ''ComicBook/{{X-Men}}''
** The series has dined out for years on the idea of [[FantasticRacism prejudice against mutants]] - to the point where it is the major thread of nearly every adaptation and any attempts to even ''tone down'' "Mutant Hysteria" (much less eliminate it) have been swiftly written out. When it was first conceived, anti-mutant prejudice was based on fears of mutant supremacy: that mutants like Magneto would eradicate/replace normal humans as the next stage in evolution, especially since anyone's child could be a mutant. As currently written, the FantasticRacism is apparently so ingrained that it has become even stronger in the face of the mutant population being BroughtDownToNormal in the Decimation arc.
** Another Plot Tumor could be the whole idea of the mutant-hunting robots, the Sentinels. The anti-mutant groups have no problem with gigantic robots, filled to the brim with all weapons of mass destruction, roaming the world in search of mutants and not stopping until they found even a mutant with the power of, oh say, glow in the dark, and destroying everything and everyone in the way until the mission is done?
* TheFlash and the Speed Force. The Speed Force started as a way of ArcWelding all of the unrelated super-speedsters while providing them with a universal HandWave for the ways that they make physicists cry. It eventually gained enough properties, applications, and relevance that it now dominates the Flash mythos.
* Many heroes with power sources that can be even remotely anthropomorphized, with the power source becoming used in more and more story elements instead of just being left in the background. For example, a lot of recent [[{{Shazam}} Marvel Family]] stories are more about the Wizard and/or the gods who empower Captain Marvel and less about the Captain himself, GreenLantern comics are frequently dominated by the Guardians and Lantern politics rather than heroics, and Comicbook/AnimalMan eventually started drowning in "the Red" (which eventually led to Animal Man ditching superheroics completely in favor of animal activism).
* In one issue of ''TheAuthority'' Swift has a one night stand with Grunge from ''{{Gen 13}}''. Later this become source of drama between Grunge and his girlfriend in ''{{Gen 13}}''.
* In the ''[[Comicbook/ArchieComicsSonicTheHedgehog Sonic the Hedgehog]]'' comics, this happened when Knuckles' own series was cancelled and all of its characters and plots got reincorporated into the main book. As a result, for a time the comic was more about Knuckles than it was about its title character. Eventually, however, the excess characters were written out ([[PutOnABus one way]] [[KilledOffForReal or another]]) and the plot lines either tied off or more evenly merged with the main series. Now, Knuckles only gets central focus when it helps contribute to the main plot.
[[/folder]]

[[folder: Literature ]]
* The HolyGrail in [[KingArthur Arthurian literature]] grew to be the entire raison d'etre of the Arthurian Court, taking on aspects of various magic hamper/magic mill myths, and creating a mythological snarl whose origins modern scholars are nowhere close to deciphering.
* When J.R.R. Tolkien was writing ''Literature/TheHobbit'', he was also [[WorldBuilding designing the fantasy world of Middle-Earth]] in his spare time, just for fun. For his own amusement, and to flesh out the world of ''Literature/TheHobbit'' a little more, he put a few references to Middle-Earth into the book, but he wasn't seriously thinking about adding hobbits to his private WorldBuilding project. However, when he decided to write a sequel to ''Literature/TheHobbit'', the Middle-Earth references increased exponentially, to the point where the book (''Literature/TheLordOfTheRings'' by name) was as much a sequel to ''TheSilmarillion'' (at that time unpublished) as it was to ''Literature/TheHobbit''. Some ''massive'' amounts of {{Retcon}}ning were needed to make the two stories fit into the same setting.
** Another, perhaps even more surprising, Tolkien example: The One Ring (and Gollum). In Tolkien's first version of ''The Hobbit'', Gollum ''willingly handed over'' the Ring to Bilbo as a prize for besting him in the riddle contest, it was just a plot point to give Bilbo the invisibility powers. Tolkien had to back and make Gollum far more sinister and un-sportsmanlike about the whole thing AND add in that he freaked out about losing the Ring, considering that in ''The Lord of the Rings'', the importance of the Ring has swollen so much that the story pretty much entirely revolves on the corrupting power of the One Ring and Middle Earth is inadvertently ''saved'' because of Gollum's need to have his precious.
* In ''Literature/HonorHarrington'', Manpower Incorporated, a genetic slavery organization, was originally a minor background detail, with the focus of the series being on the increasingly escalating war between the protagonist Star Kingdom of Manticore and the antagonistic People's Republic of Haven. Starting around the tenth book in the series, however, the Mesa system, which is both home to and arm-in-arm with Manpower, became the BigBad of the story. The shadowy cabal that controls Mesa was eventually revealed to be closing in on the end of a centuries-long plan to control the known galaxy via long-term political and military manipulation of every other government, all so they can reintroduce widespread genetic engineering to the galaxy and eliminate any opposition to it.
[[/folder]]

[[folder: Live Action TV ]]
* ''Series/DoctorWho'' has a couple of examples.
** The Cybermen's allergy to gold went from "could be choked by powdered gold dust" to "tossing a gold coin at them is like shooting Kryptonite bullets". When the new series reintroduced them, this tumor was quietly excised. [[AllThereInTheManual Supplemental material]] mentions that the allergy to gold was discovered early in the Cybermen's R&D process and eliminated then.
*** And then brought back in a new series episode, where a golden ticket temporarily disables a cybernetic implant capable of overpowering the Doctor himself.
** In the sonic screwdriver's original appearance in ''Fury from the Deep'', and later in ''The War Games'', it was used ''for unscrewing things''. It only gradually became a do-anything device. It was actually written out of the show because it was becoming a DeusExMachina, before being reintroduced for the Eighth Doctor... and becoming a DeusExMachina. It got so bad that they had to introduce a ''second'' DeusExMachina ("deadlocking") to counteract it. And now it doesn't work on [[WeaksauceWeakness wood]] either.
* On the second season of ''Series/{{Grimm}}'', Juliette's amnesia and subsequent relationship and mental health issues became the B-plot of the entire season, which was odd, since she was a relatively minor character in the first season. Some found it incongruous, because the show is otherwise mostly about magical creature-people, their society and how they interact with (and sometimes threaten) humans.
* The Kromaggs on ''Series/{{Sliders}}''.
* When Ellen [=DeGeneres=] came out of the closet, then character on her sit-com, ''{{Ellen}}'', did the same. It had been stated that the show would continue normally and that her being gay would not take over the show. However, in the fifth and final season, half the episodes focus on it.
* The surreal comedy episodes of ''Series/TheXFiles'' became the series' plot tumour in the sixth season especially. Whereas previously there'd been two or three per season that were refreshing escapes from the show's usually dark and disturbing subject matter, it was a bit much to get a silly comedy episode every single week. (The other perspective was that this was a relief from the conspiracy arc that [[TheChrisCarterEffect didn't seem to know where it was going]], but that's a matter of fan opinion.)
* Archie Kennedy in ''Series/HoratioHornblower'' became a rather inconvenient character-flavored plot tumor (and nearly a RomanticPlotTumor if you squint) as a minor character [[CompositeCharacter cobbled together from several bookverse extras]] and who proceeded to swell vastly in importance as the highly non-canon best friend of a hero not known for having extroverted besties. [[spoiler: Was forcibly excised when the Forester estate demanded that the character be killed at the end of the 6th film in order to refocus the film series on an appropriately introverted Horatio.]]
* Due to ExecutiveMeddling, WritingByTheSeatOfYourPants and a host of other factors, the second season of ''TwinPeaks'' ended up being dominated by several inconsequential storylines that had originally been planned as minor subplots or filler - the Miss Twin Peaks pageant, the adoption of Nicky, and so on.
[[/folder]]

[[folder: Machinima]]
* The Freelancer program of ''Machinima/RedVsBlue'' began simply as independent soldiers who worked for the paying side and to introduce the AI programs. The mini-series Out of Mind expanded this to being a special program to combine [=AIs=] with soldiers and the [[AIIsACrapshoot AI revolt]]. The Recovery One and Recollection trilogy further expanded it to be not only a program designed to win the great war with unscrupulous methods, but the cause of the Red vs. Blue war and all the events of the first five seasons as an extended Freelancer training scenario. Finally, the ninth and tenth seasons had the plot equal parts silly comedy and the darker Freelancer backstory.
[[/folder]]

[[folder: Multiple media ]]
* ''Franchise/StarTrek'' across its many incarnations and writers has had a lot of these.
** The "Brain Bugs" from ''[[Film/StarTrekIITheWrathOfKhan The Wrath of Khan]]'' went from a moderately small concept to a much larger one as [[RunningTheAsylum successive generations of fans got a hold of the writing jobs]]. Incidentally, [[http://stardestroyer.net/Empire/Essays/BrainBugs.html "Brain Bugs"]] is sometimes used as an interchangeable term for Plot Tumors; whether or not this is influenced by ''Wrath of Khan'' is up for question.
** The Jefferies Tubes started out as fairly realistic maintenance tunnels that the odd tool or piece of equipment were in. This is realistic because sometimes with complicated engineering not everything is within arm's reach. These mutated over generations to labyrinths of tubes where everything important was kept - Fair to say no engineer would design something [[MalevolentArchitecture this malevolent]].
*** The ''Series/StarTrekVoyager'' episode "Macrocosm" {{lampshaded}} this when Janeway gives the Doctor a rather complex set of directions through the tubes and the Doctor asks who designed the ship.
*** Parodied and Lampshaded in ''Film/GalaxyQuest''.
---> "Whoever wrote this episode should '''''DIE!'''''"
** The Borg started out as technophiles who were only interested in stealing interesting technology and ignored life forms unless they became a danger, but they gradually mutated into ''Night of the Living Dead''-style zombies. The first trip aboard a Borg Cube showed that Borg reproduced naturally and put implants into their infants. Meanwhile Picard was chosen to become Locutus as a mouthpiece for the Borg to announce their intention to conquer and enslave humanity. By ''First Contact'', they were able to assimilate on the fly, injecting nanites directly into people to begin the assimilation process (though full assimilation was more involved). By the time ''[[Series/StarTrekVoyager Voyager]]'' encountered them, they were primarily interested in assimilating life forms regardless of their technological level, and eventually the Borg were revealed to be unable to actually understand ''anything'' without assimilating it.
** The Vulcans have also gone through this. One of the complaints about the Vulcans on ''Enterprise'' was that they were portrayed as capable of deceit and underhanded behavior, the complaints arising because people took one character, Spock, who was in fact notably ''atypical'', and used him as the archetype for an entire species. However, when you look at how Vulcans were portrayed in the canon, you saw Vulcans acting in quite un-Spockish ways, even as far back as his "wife" in the original series who manipulated things to get out of her arranged marriage.
** Transporters were created as a last-minute cost-cutting cop-out to prevent expensive effects shots of shuttles landing on planets, but soon became a rich source of plots, with whole episodes centered on [[TeleporterAccident the zanier aspects of their operation]], even though the [[http://www.xibalba.demon.co.uk/jbr/trek/7.html unintended applications]] make them outrageous and are best ignored to begin with. See MisappliedPhlebotinum.
** Perhaps the biggest one was the way the Prime Directive grew in importance until ''[[Series/StarTrekEnterprise Enterprise]]'' was doing an episode where the FamilyUnfriendlyAesop was that the only moral thing to do was to stand by and let an entire species of advanced, peaceful aliens die out when you could easily save them. Worse, the Prime Directive didn't even exist at the time that series was set. That species was allowed to die out because the character had somehow got it into his head that [[YouFailBiologyForever "evolution" is some kind of omniscient God who must not be disobeyed]].
** The entire Dominion War arc that practically ''was Series/StarTrekDeepSpaceNine'', brought on apparently to [[DuelingShows compete with]] ''Series/{{Babylon 5}}'''s series-spanning space war that aired at the same time.
* ''Franchise/StarWars'' has several examples.
** It can be argued that [[EnergyWeapon lightsaber]] combat qualifies, as it was never paid that much attention in the original trilogy compared to the prequels (and in the ExpandedUniverse, there are seven ''forms'' of lightsaber combat, each explored in detail).
** Boba Fett -- starting with a [[EnsembleDarkhorse non-notable background character]] with almost no dialogue, whom the audience liked for his "cool" armor, and ending up with the Mandalorians, an entire society of [[ProudWarriorRaceGuy proud warrior race guys]] like him, who have played a major role in at least two galaxy-spanning conflicts to date and basically became the ''Star Wars'' answer to the Klingons (not to mention the original source of the stormtroopers!).
** The ''Star Wars'' video games, having a relatively limited amount of iconic canonical material to draw on, have become almost comical in the way various memorable elements of the movies show up over and over again in different, unrelated games. For instance, there are now, in the non-canonical parts of the EU at least, at least five different sets of Death Star plans that have been stolen five different and mutually exclusive ways by five different heroes or sets of heroes. And even with what ''is'' still canonical there's still at least two versions of how each half of the plans were stolen, both featuring entirely different people doing the work.
*** It's notable in that, for a period of time, the games were generally consistent with one other (besides the above), where if something appeared in one game it would look the same as it did if it reappeared in another - when games started getting {{licensed|Game}} out to other developers was when [[ChaosArchitecture they took their own spin on locations]] and plots and nothing really matched up anymore.
** See also how thanks to the video games' endless reliving of [[Film/TheEmpireStrikesBack the Battle of Hoth]], what was, in the movies, a one-time, fairly cool longshot that happened to pay off - the snowspeeder managing to trip the AT-AT with a tow cable - has now become the de rigeur, recommended means for killing [=AT-ATs=].
** Force Lightning is used six times in the six movies - three times by Palpatine (''Return of the Jedi'', twice in ''Revenge of the Sith'') and three times by Dooku/Darth Tyrannus (all in ''Attack Of The Clones''). Both major league Sith Lords. In the games, anyone who has a smidge of Dark Side can throw lightning around with impunity [[http://jediknight3.filefront.com/potd/40774 and on a vastly greater scale, too]].
** Hoth is an utterly unremarkable, nigh-uninhabitable snowball of a planet whose sole significance was a major Rebel base built there specifically ''because'' it's the last place anyone would look. And yet almost every Star Wars game since ''Literature/ShadowsOfTheEmpire'' features a Hoth level. Which would be excusable if the game, like ''[=SotE=]'', were set during Episode 5 or between it and 6 - most are not ''and shoehorn in a Hoth level anyway!''
*** It gets to the point that, in ''[[VideoGame/DarkForcesSaga Jedi Knight: Jedi Academy]]'', when Luke is sending characters out to investigate whether the bad guys have absorbed dark Force energies from various planets, he's certain that the player character will find them on Hoth because of that ten-second vision of Obi-Wan from ''Empire'', and at the same time certain that [[TheRival Rosh]] ''won't'' find them at ''the destroyed remains of a planet that was basically Emperor Palpatine's home world during the civil war''. [[spoiler:It's no surprise that the cult is at both planets, though at the very least they seem to be on Hoth mostly to look up the old flight logs to find out about other planets with strong Force auras Luke had been to - say, Dagobah.]]
*** Lampshaded in ''VideoGame/StarWarsTheOldRepublic'', where during the Hoth mission, the player states s/he has never heard of the planet. The Republic, apparently, is only there to salvage wreckage from a battle which occured nearby... The Empire's agenda is tying up Republic forces on a meaningless iceball.
** Similarly, it seems everything visits Tatooine, which was introduced as [[NothingExcitingEverHappensHere the middle of nowhere]].
** Cutting off arms and hands is a running theme in the films. In the Expanded Universe, it's given a whole backstory about how it's considered a traditional display of lightsaber skill. There's even a catalog of different versions, each with a different name.
** In the end of ''Film/ReturnOfTheJedi'', we see Anakin's spirit standing alongside Jedi, meaning he achieved ''spiritual'' redemption. There is no indication leaving the Dark Side amounts to anything more. A KarmaHoudini, maybe, but at least there is no indication of it going ''beyond'' the Karma - in the expanded universe he's still remembered pretty much solely as the ultimately-evil Dark Lord of the Sith. For ''absolutely everyone else'' in EU material, going back to Light seems to give an instant ''legal'' amnesty and a clean slate, no matter how many billions they may have murdered in cold blood beforehand.
[[/folder]]

[[folder: Newspaper Comics ]]
* Few people realize that ''ComicStrip/BeetleBailey'' was originally about the title character attending college. Him joining the army (in 1951) was originally going to be just a one shot story. He was in college for only ''six months'' before leaving forever. In this case, it was (arguably) a change for the better since the military focus made it far more popular and commercially successful.
* ''ComicStrip/{{Blondie}}'' was originally about a flapper girl from the twenties of the same name. After the Great Depression hit, the focus of the comic turned to domestic comedy involving her marriage to her inept comically-oversized sandwich-eating husband, Dagwood Bumstead. Dagwood himself originally came from wealthy roots - to mark the shift in focus, he was disowned by his family and his wealth for marrying below his social class and thus had to enter the blue-collar working world that he was unprepared for while Blondie shifted from gold-digging flapper to responsible and caring matriarch.
[[/folder]]

[[folder: Theatre ]]
* In the original play ''Theatre/{{Chicago}}'', Roxie merely mentions vaudeville as what she intended to do after getting out of jail. In the musical, vaudeville is the lifelong ambition of both Roxie and Velma, and every musical number has a vaudeville motif. Women who murdered their husbands for cheating only became a major subject in the musical, which not only added the "Cell Block Tango" but completely rewrote one minor character who appears later to fit that theme.
[[/folder]]

[[folder: Toys ]]
* Pretty much every single ''Franchise/{{Transformers}}'' series since about 1992 where Takara (the Japanese toy company that shares the rights to the ''Transformers'' brand with their American partner Hasbro) had a major say in the direction of the toyline/story development has over-emphasized the role of Convoy (better known as Optimus Prime outside Japan) and his derivatives (Hasbro, on the other hand, despite also putting an "Optimus"/"Prime" character/toy in every series whenever possible, puts a little more emphasis on character diversity). This is particularly glaring in short-lived toy-only lines with no television show to back them up, which will often start with a new Convoy toy... then ''maybe'' a different character as the second toy if they're really lucky, or ''another'' Convoy-related toy of they're not so lucky... and then the line ends and gets replaced by a new line that starts with the next Convoy all over again. The most noteworthy example would be the "Robot Masters" line from 2004, which, during its 25-toy-run, had no less than ''seven'' toys with the word "Convoy" in their names (including redecos). One of these "Convoy" toys was even a retool of a ''Megatron'' toy and was intended to actually ''be'' a form of Megatron.
** Prime's role can be over-emphasized in America, too; The most recent [[TransformersFilmSeries movie]] had a villain who could only be defeated by a Prime.
** A more notable example in the Transformers mythos: The Autobot Matrix of Leadership. The term "matrix" originated with Optimus's "creation matrix" in the comics, in which it was simply used to create new [[MerchandiseDriven toys]]. It was then introduced into the movie with its current title, serving only as a MacGuffin to defeat Unicron (note that prior to Unicron mentioning it, Megatron had ''absolutely no use for it''); it quickly became the central [[GreenLanternRing do-anything power source]] and all-purpose MacGuffin for the cartoon. Several series have even had Megatron and other Decepticon leaders dip into MotiveDecay by having them all lust after the Matrix. In ''All Hail Megatron'', Megatron's acquisition of the Matrix was treated as "game over" for the Autobots, and [[TheStarscream Starscream]] was able to win over the ''Decepticon'' army just by possessing it.
** Considering that in everything except the G1 cartoon, Beast Wars and Beast Machines, the Matrix contains part of the essence of the Transformer god and creator, Primus, it's understandable that the Decepticons would want it. Still though...
** For that matter, the Primus/Unicron conflict. In G1, Unicron was "merely" a humongous planet-devouring Transformer, who got defeated in the very movie he appeared in. Eventually, however, he was retconned into a "multiversal singularity" existing ''in every Transformers continuity at once'', along with his good counterpart, who was introduced into the mythos even later. Eventually, fans got so sick of Primus and Unicron that ''TransformersAnimated'' very pointedly avoided mentioning either of them.
** Sparks. They were a creation of ''Beast Wars''; nothing in G1 pointed to the idea that an actual object was required to keep a Transformer operational, any more that one normally would for a machine. The Dinobots and Combaticons were built on site with no access to any "life-giving" entities, the Stunticons and Aerialbots were animated by Vector Sigma, and Optimus Prime was simply ''repaired'' into full working order by a random Quintesson after dying (twice, at that). By the time of the movie and ''Animated'', the BW idea of sparks firmly secured their place in the mythos, to the point of postulating that all Transformers were creations of the [=AllSpark=].
[[/folder]]

[[folder: Video Games ]]
* ''NeverwinterNights2'', starting with the Ember Trial, seems to force one quest on the player after another, leading to the question "Whatever happened to going to the Jerro Estate?", it takes most of part II for the answer to that question
* The Insult Swordfighting in ''VideoGame/MonkeyIsland'' was meant to be a parody of the [[YouFightLikeACow witty banter]] found in high adventure movies, but by the time ''VideoGame/EscapeFromMonkeyIsland'' came around, there's apparently an Insult version of nearly every sport available floating around the Tri-Island Area.
** ''Escape'' also included TheReveal that [[spoiler:Herman Toothrot is H. T. Marley with EasyAmnesia]].
* Sometimes the earlier games in ''Zelda'' series seem to be set in an almost separate universe than the more modern ones. Originally, the Triforce was a mysterious triangle that granted magical abilities, and there were only two of them, not three. Come ''VideoGame/TheLegendOfZeldaALinkToThePast'', however, and it's the CosmicKeystone of the entire ''Zelda'' universe with omnipotent wish-granting and reality-warping powers. The significance of the Triforce mark was also different. From ''VideoGame/TheLegendOfZeldaOcarinaOfTime'' onward, the Triforce mark on one's hand signified which piece of the Triforce one had and would glow when its power was being used. The first appearance of this mark was in ''VideoGame/ZeldaIITheAdventureOfLink'', and it showed up on Link's hand before he even had the Triforce of Courage. It just marked him as the hero destined to claim it.
** Likewise, the Master Sword being the only sword able to kill Ganon is a relatively recent idea. In its first appearance in ''A Link to the Past'', the Master Sword was a powerful weapon to defeat evil, but in order to kill Ganon you had to stun him with the Master Sword, then actually harm him with a Silver Arrow. After ''Ocarina of Time'' the relationship has been reversed, and the Light Arrows are needed to stun Ganon so you can harm him with the Master Sword. Sometimes you don't even need the arrows at all. The Master Sword meanwhile has been given increased importance, and it's a Comic Keystone just as important to the world of Hyrule as the Triforce now. Although, in ''Ocarina of Time'', it was required only to seal him away; you're able to harm Ganon with the [[InfinityMinusOneSword Biggoron Sword]].
* The ''Franchise/MetalGear'' games were once about bipedal nuclear tanks but ever since ''VideoGame/MetalGearSolid2SonsOfLiberty'' introduced the Patriots, everything, even retroactively, has something to do with them.
** In the earlier games, UnusableEnemyEquipment was handwaved by the fact that the weapons were keyed to their users via {{Nanomachines}} and won't function for anyone else (which still doesn't explain why enemy guns couldn't be used in ''Videogame/MetalGearSolid3SnakeEater'', which is set in the 1960s and should predate that technology). In ''VideoGame/MetalGearSolid4GunsOfThePatriots'', the entire plot revolves around the weapon-identifying nanomachines (hence the subtitle) and the computer system that regulates them. Ironically enough, this is also the game that introduces the ability to take weapons from their keyed users and unlock them for your own use.
** By a certain scale, nanomachines started to be responsible for basically maintaining the entire modern world order and was probably intended to be used eventually on every civilian in the world along with government officials. The Patriots were even revealed to basically just be the AI System regulating said nanomachines, with the original human ones being reduced to psychotic messes, rebel leaders, persistent vegetables, or just plain dead.
* The Cerberus group in the ''Franchise/MassEffect'' series. Initially it was just a shadowy organization with a few bases operating on uncharted worlds, with substantial influence but no indications of connections to major plots. Come ''VideoGame/MassEffect2'' they're a central part of the plot, and in ''VideoGame/MassEffect3'' they're a major military power capable of seriously challenging the Alliance and sabotaging the war effort against the Reapers. They also form a central role in three of the four ExpandedUniverse novels.
* Aku Aku in the first two ''Franchise/CrashBandicoot'' games was just a silent benevolent spirit in a mask that provides the hero protection and temporally invincibility and had absolutely no relevance on the plot. Comes ''[[VideoGame/CrashBandicoot3Warped Warped]]'', Aku Aku's role [[AscendedExtra is expended as Crash and Coco's mentor]], [[SuddenlyVoiced provides him a voice]] and his EvilTwin Uka Uka is introduced to which former BigBad Cortex is DemotedToDragon. Not only it contributed at Cortex VillainDecay but also derails the plot into a strife between two opposites gods in which both Crash and Cortex are pawns in furthers games.
* Prior to WorldOfWarcraft, the Old Gods' role in the main story lines was minimal at best; it was known they existed and had a significant impact on the setting, but were a footnote compared to more immediate evils such as the Horde, the Burning Legion, and the Scourge. Come the MMO however, and their role has expanded exponentially each expansion, to the point that by ''Cataclysm'', they were basically the main antagonists alongside BigBad Deathwing, and in ''Mists of Pandaria'', their influence is what caused the creation of the Sha, the evil beings that are antagonizing the continent and whose power is harnessed by people such as Garrosh to fuel their plans.
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* ''Webcomic/SluggyFreelance'' has had a couple of these, thanks to being KudzuPlot incarnate.
** The Dimension of Pain was originally just a concept that fueled roughly a week's worth of strips. Then Pete Abrams decided to make it a RunningGag, having the Dimension of Pain demons show up each Halloween to claim Torg's soul. Each Halloween arc got longer than the last, and eventually the demons caught on so much that they were made the stars of their own [[BSideComics B Side Comic Strip]] "Meanwhile in the Dimension of Pain." Then eventually even ''that'' wasn't enough, and the Dimension of Pain demons became the main antagonists of the massive "That Which Redeems" arc.
*** Although Pete Abrams has stated he knew the demons would be invading the Dimension of Lame when he left them a potential means to do so, right in the first story.
** There's also Bun-Bun's grudge against Santa Claus. At first there were just a handful of strips around Christmas each year where Bun-Bun would try (and fail) to kill Santa Claus. The feud kept escalating, however, with more and more side characters (the Easter Bunny, Santa's black ops elves, aliens with a weakness against Nerf) getting involved, until it eventually exploded into Bun-Bun amassing an army and going on a holiday killing, world conquering rampage in the three month long "Holiday Wars" saga (which itself launched the even longer "Oceans Unmoving" arc).
** Both of these Plot Tumors, however, seem to have been successfully removed. The Dimension of Pain has not been seen for years, ever since "That Which Redeems" concluded. And, while Bun-Bun still makes the occasional attack on Santa Claus, holiday figures and black ops elves have long since ceased to play a prominent role in the story.
*** Although with the 4U city mutants and the mutagen causing squid on a stick, the Dimension of Pain looks to be moving back towards being important. Timeless space also managed to be fit in.
* ''{{Concession}}'' started off as a comic strip about a bunch of anthropomorphic characters who worked at a concession stand at a movie theater (The author actually based it around the stupidity he experienced, working in customer service is a good way to get material for comic strips). But in the later comics...you'll not really see that much about an actual concession stand. For awhile, the actual concession stand was more or less put to the side, and until it got wrapped up in the massive Plot Tumor, it didn't even play a role beyond the occasional appearance of a main character who was still employed there. Immelmann has actually ''admitted'' that it's only about concession stands InNameOnly and centers around the character Joel and his plot. It even says so right in the "About" section.
* In ''SamAndFuzzy'' the original comic was mainly a slice of life style that was mainly a gag a day style. Then ninjas were added. And a demonic refrigerator. After that, things got weird. Now the whole story revolves around the weirdness and the weirdos and pretty much every arc has ninjas in it due to Sam becoming the Ninja Emperor.
* The "Patriarchy" in Webcomic/{{Sinfest}} during fall of 2011 quickly grew to overtake the strip, turning the focus to the actions of Trike Girl and the ramifications of said actions on the world. Webcomic/{{Sinfest}} runs into this trope a lot, due to the author [[WritingByTheSeatOfYourPants writing by the seat of his pants]]. "Patriarchy" is notable for taking over so much in such a short time period, but there are plenty of other examples:
** Possibly the first was the Devil's "crisis of faith", which spun so far out of control that the author didn't know how to end it. Big D was AWOL for several real-world months before it was revealed that he just went on vacation.
** The "Reality Zone" was introduced for a one-off Sunday strip, then became a recurring plot element.
** A 2010 storyline had Squig becoming a hobo and WalkingTheEarth meeting all sorts of [[ShoutOut weird cameos]], eventually winding up lost in a desert and pining for home. The author was eventually able to tweak an unrelated storyline to get Squig home.
** And the big one: The Fuschia/Criminy subplot, which became the strip's chief attraction for awhile and still pops up frequently to mess with the status quo.
* The trolls in ''Webcomic/{{Homestuck}}'' could be considered this. Most fans don't even remember that they didn't appear until Act 3 and they were retroactively added into the earlier story through flashbacks.
%% Title is not an example; Sburb was always going to be the focus, Homestuck was just chosen as a name because it was more distinctive than just "Sburb".
* The same thing happened in ''ProblemSleuth'', the comic to which ''Homestuck'' is a SpiritualSuccessor -- originally, the imaginary world was just a side-thing used to solve some puzzles, but eventually the action moved there entirely to the point where it was hard to remember that it wasn't really real.
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