For almost a full season, the plucky folks who [[WeHelpTheHelpless help those in need]] have defeated {{Monster of the Week}} after {{Monster of the Week}}, protected the space-time continuum from invading aliens, stopped an {{Antimatter}} explosion from removing North America from the face of the planet, and saved the [[OurPresidentsAreDifferent President]].

Now, however, they are vexed by someone who seems to know their strengths, their weaknesses, and everything in between. They're outmaneuvered, outnumbered, and outsmarted. The season ends with the villain gloating over them, saying their most dangerous tasks up to that point have simply been tests he has engineered.

The Arc Welder cometh.

ArcWelding is a retroactive form of ContinuityCreep that occurs when a series which has heretofore been episodic {{retcon}}s itself so that it's all linked in a StoryArc. The most common approach to ArcWelding is probably when one antagonist, hidden up to that point, is responsible for all the major threats the protagonists have faced thus far.

Alternatively, several arcs might be revealed to part of a larger MythArc. This is rarer, possibly because it's harder to do well and shows that would benefit from it might not last long enough to play out the {{retcon}}.

Arc welding is different from StoryArc or MythArc because it is always retroactive. Series that start out with a StoryArc or MythArc already in place generally aren't welded. If something were already part of an arc, it wouldn't need welding.

The creators may be simply putting unrelated crap together for the sake of using up the budget now that they're renewed. It can be also be a stroke of brilliance, the creators now realizing an underlying theme of their work on the series thus far. Lastly, ArcWelding itself can be plotted from the beginning, as TheReveal.

Compare TheMoorcockEffect, which ties together different series, and MidnightOnTheFiringLine, which retroactively ties a seemingly unrelated event into an existing StoryArc. See also MetaOrigin.
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!!Examples

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[[folder: Anime and Manga ]]

* In ''{{Bleach}}'', a large number of seemingly incidental minor villains and events which occured throughout the series are [[TheReveal revealed]] to have been caused by the BigBad in season 3.
* A minor {{Naruto}} example: A {{Filler}} episode revealed that Mizuki attempted to steal the scroll of kinjutsu on [[BigBad Orochimaru's]] orders. This revelation occurred around episode 160 or so, and Mizuki stole the scoll in episode 1.
** A major {{Naruto}} example: [[spoiler: Uchiha Madara is apparently largely responsible for the origins of Naruto, Sasuke, the Leaf Village, and potentially many more elements of the story.]]
*** Not to mention that the [[spoiler:Sage of the Six Paths, besides being the first guy with the Rinn'egan as well as creating the whole ninja world as it is, also is source of the Senju and the Uchiha clans, the guy who split the almighty ten-tailed beast into the more familar one to nine-tailed beasts, creating the MOON on the whole process. Busy guy.]]
* In ''SailorMoon'' DIC tried to do this to all the bad guys that appeared during the second season. Then later [[spoiler: the original creator did this with the bad guy ''Chaos'' at the end of the series who was supposedly responsible for the appearance of all the previous {{Big Bad}}s]]
** I took it less to mean that it was all some preplanned conspiracy by Chaos, and more that Chaos is the [[ChaoticEvil physical manifestation of evil]], therefore all the previous {{Big Bad}}s (being evil) were manifestations of itself.
* ''TenjhoTenge''. Every bad thing that ever happened to anyone turned out to be the work of [[spoiler:the protagonist's dad]]. Even the stuff that happened centuries ago.
* ''YuGiOhGX'' has this with season four, where it is revealed that the release of the Sacred Beasts, the Light of Destruction, and Yubel's schemes were all used by the final {{big bad}} Darkness for...something.
* Chapter 170 of YuYuHakusho confirms that [[spoiler:everything that happened in the series was a result of ''King Enma's'' actions. He created falsified reports of demon crimes in the human realm, let evil demons into the human realm, and even brainwashed innocent demons for his Spirit Detectives to finish off, increasing Enma's reputation within Spirit World, since the Spirit Detectives were under his command. This gave Enma the authority to erect a barrier between the three realms, as supposed demon "crimes" would create the illusion that this was a righteous act, and not totalitarian, since the spirit energy unused by humans was a valuable resource to the Spirit World.]]
* ''PokemonSpecial'' tends to do this. Both good guys and bad unexpectly show up in later arcs with [[XanatosGambit Xanatos Gambits]]ahoy, but FLRG and Emerald take the cake. FLRG was a result of all the previous arcs while Emerald was a direct consequence of FLRG.

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[[folder: Comic Books ]]

* {{Cerebus}} did this ''a lot''. There were very few minor, throwaway characters. Just about anyone who talked to Cerebus at some point is revealed to be important to the plot somehow.
* The comic ''{{Transmetropolitan}}'' starts its MythArc with issue #13 (the Year of the Bastard storyline), but starts ArcWelding sometime around issue #30. Though it takes a while, events from the first issue are eventually revealed to have had an impact all the way to the last.
* One of the highest quality examples of this is [[DonRosa Don Rosa's]] ''TheLifeAndTimesOfScroogeMcDuck'' series, which takes every random reference CarlBarks ever made to Scrooge's past and puts together a comprehensive, sensical and engaging character biography. If the Eisner award is to be believed, it ''really'' worked.
* GeoffJohns has been doing this in the GreenLantern books, introducing an "emotional spectrum" that people can draw power from. The Green Lantern Corps fall square in the middle at green willpower; with red rage, orange avarice and yellow fear on one side and blue hope, indigo compassion, and violet love on the other. Not only are existing characters being tied into the spectrum (villainess Star Sapphire tapping into love energy, for example), but Johns is starting to establish whole rival corps for each (like yellow ring-wielder Sinestro starting his own Sinestro Corps of fear).
* When Marvel finally decided to fully reveal [[Comicbook/XMen Wolverine's]] [[ExpansionPackPast long and complicated]] origin story, it turns out that just about every bad thing that ever happened to him was orchestrated by a single figure known as Romulus.
* In ''[[UltimateMarvel Ultimate]] {{Spider-Man}}'', several {{Batman Cold Open}}s involving him fighting some villain who attacked "Roxxon Industries" were welded together when the CEO of that company (a person rather lacking in common sense) hired some mercenaries to bring him in for questioning about why he was fighting those people.

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[[folder: Literature ]]

* In ''TheDresdenFiles'', Harry comes to the conclusion that the problems he has been dealing with since book one is the work of a group that he calls the Black Council. He first voices his suspicions at the end of the second book (''Fool Moon''), but isn't sure of his conclusion until book eight (''Proven Guilty'').
** Butcher has stated [[http://www.jim-butcher.com/books/dresden here on his site]] that he has plans running into the lower twenties for the series. Book eight out of an estimated twenty-three is still pretty early for a series.
* SherlockHolmes. It was revealed in the first story to feature Moriarty that the master villain had been implicit in several of Holmes' cases before, but had simply never been mentioned until now.
* ''[[ASeriesOfUnfortunateEvents Lemony Snicket: The Unauthorized Autobiography]]'' played this for laughs. Most of the book consists of explaining how everything that had happened up to that point in the series was related to the VFD in various ludicrous ways.

[[/folder]]

[[folder: Live Action TV ]]

* One of the more explicitly identified examples of ArcWelding comes from ''{{Angel}}'''s fourth season, where Skip notes that everything that's happened to Angel and company for a very long time -- Angel's ensouling, Cordelia's ascension, Fred's being trapped in Pylea, Lorne's banishment ''from'' Pylea -- were all part of a XanatosRoulette.
** A smaller version then happened in the next and final season: faced with ExecutiveMeddling to be more episodic without any arcs, after finding out the show would be canceled anyway Joss spent the last two episodes revealing that many of the villains over the season were part of a group called the Circle of the Black Thorn, then having the heroes kill them all.
* ''StarTrek'': ''DeepSpaceNine'' does excellent examples of {{Story Arc}}s and also of ArcWelding. Starting in mid-season 2 and possibly earlier, the seeds are planted in an almost offhanded manner for the coming storyline. Then, at the end of season 2, it's all revealed to be part of a big plot that isn't concluded until the final episode of the final season -- which itself is the finale of a 9-part arc within the MythArc of the show.
**Also, ExecutiveMeddling resulted in the Klingons returning to their [[StarTrekTheOriginalSeries Kirk-era]] level of villainy just as the Dominion was planned to take center stage. ThePowersThatBe did the best they could, and made it fit at the end by making Changeling manipulation responsible.
* ''TheXFiles'' did this in later seasons, with dormant alien DNA supposedly accounting for much of the apparently Earthbound paranormal activity Mulder and Scully investigated.
* Arc Patching, if not ArcWelding, was done in ''VeronicaMars'', when the season 2 bus bombing storyline wrapped up. The perpetrator was revealed to also have raped Veronica at Shelly Pomroy's party, a storyline thought to be wrapped up in season 1 as being not rape, but mutually drugged-up semi-consensual sex. This explained Veronica's chlamydia, despite her having only two (or, as TheReveal made plain, actually three) sexual partners and presumably using protection, the existence of which was used to paint Veronica as a slut and therefore untrustworthy in the trial of Aaron Echolls. The blatant illegality of delving into her medical records for some reason ''not'' resulting in a mistrial is another debate entirely.
* ''{{Stargate}}'', with the Ori plot continuing for two seasons after it was meant to be cancelled with a big chunk of NeglectfulPrecursor plot shoved in for good measure.
* At the end of the first season of ''TheSopranos'', Big Pussy has vanished. [[TitleDrop No one knows anything]]. The writers of the show were just going to let it go at that--people do, indeed, vanish with no explanation, though it's rare. However, when they heard how the fans were wondering what happened to him, they welded Pussy into the story of Jimmy's being TheMole, with him being a second one.
* ''DoctorWho'' did a bit of ArcWelding near the end of its original run, when Ace's involvement with the Doctor between ''Dragonfire'' and [[TheCurseOfFenric ''The Curse of Fenric'']] (eight multi-part stories over three seasons by that point) was revealed to be part of Fenric's XanatosGambit to trap the Doctor (moral of the story: don't try a XanatosGambit against a {{Chessmaster}}). It also provided a [[HilariousInHindsight bit of disappointment]] for Classic Who fans when the series returned and [[ArcWords "Bad Wolf"]] turned out not to be a returned Fenric.
** Series 4 of NuWho also arc welds sever standalone episodes, revealing they had something to do with [[spoiler: the Daleks ending the universe]] and [[spoiler: the creation of 10.5 (or Handy, or Blue!10, or whatever FanNickname you prefer)]].
* {{Stella}} used [[YouLookFamiliar one frequent actor]] to set up an arc for the series.

[[/folder]]

[[folder: Video Games ]]

* The first 3 ''MetalGear'' games were mostly self-contained. ''MetalGearSolid 2'' introduced [[AncientConspiracy The Patriots]], whose operatives [[spoiler: Revolver Ocelot]] and [[spoiler: George Sears]] were largely responsible for the events of ''Solid''. ''MetalGearSolid 3's'' place in the Patriots arc is not immediately apparent. ''MetalGearSolid 4'' wrapped up the arc, and revealed that [=MGS3=] was [[spoiler:the Patriots' origin story.]]
* In ''GuildWars Nightfall'', it's revealed that servants of Abaddon were responsible for driving the Charr into human lands in Tyria, leading to all the major background events of the ''Prophecies'' campaign; another servant led to the downfall of Shiro Tagachi, the BigBad of ''Factions''.
* The {{Chzo Mythos}} tetralogy of games feature this; ''5 Days a Stranger'' was originally a stand-alone game, as was its distant sequel, but the later titles tie them together to form an encompassing story arc. Ironically, the major threat of the first two games (and an important story character in the other two) is an insane killer referred to as 'The Welder' because of the mask he wears.

[[/folder]]

[[folder: Western Animation ]]

* Interviews with the creative team behind ''JusticeLeagueUnlimited'' reveal that they were several episodes into production before they realized that they were working toward what became of the "Cadmus Arc" -- even though the final product appears seamless, with throw-away lines being ArcWelded into major clues. In particular, the penultimate episode's [[TheReveal Big Reveal]] [[spoiler: that Brainiac was possessing Luthor and influencing his actions]] was a last-minute decision, ArcWelding story elements that dated back to the previous ''Superman'' series.
** Similarly, the creators of the animated ''TeenTitans'' series waited until nearly the end of the first season to reveal the BigBad Slade's master plan ([[spoiler:turning Robin into his [[strike:[[HoYay catamite]]]] apprentice]]) because it took them that long to figure it out themselves.
* Parodied in ''CloneHigh'''s final episode, where the [[TheOmniscientCouncilOfVagueness Shadowy Board of Directors]] bring together literally every one-shot guest character or celebrity and address them as collaborators. Of course, this was mostly just so the episode could end with [[spoiler:''literally every character ever to appear on the show'' - except Scudworth - frozen inside a meat locker.]]
* Most of the middle part of ''{{Gargoyles}}'' season 2 was already An Arc, but then, during the two-part season finale, where you see Oberon's Children filing into his castle to be recognized by him back on Avalon--and you realize they're familiar to you. Odin, Anansi, Banshee, Coyote, even Anubis are all his subjects. How powerful, then, must Oberon be? This was already a StoryArc of sorts--though Angela's, Goliath's, and Elisa's adventures were episodic, they were already linked by their method of travel--but now you see it was all part of a second arc as well, to set up the season 2 finale.
* ''{{Oban Star-Racers}}'': [[spoiler:In the last episode, [[BigBad Canaletto]] gives Eva/Molly a HannibalLecture where he tells her that he's been causing every ContrivedCoincidence in her life that lead to her winning the ultimate prize, but turning down the position of Avatar so he can be freed.]].
* ''DannyPhantom'', when Danny learns about his ice powers. Seemingly coming out of nowhere, it was shown as "developing" throughout the show in the form of his ghost sense.



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