Arc Welding

For almost a full season, the plucky folks who 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 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.

Arc Welding is a retroactive form of Continuity Creep that occurs when a series which has heretofore been episodic retcons itself so that it's all linked in a Story Arc. The most common approach to Arc Welding 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 Myth Arc. 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 Story Arc or Myth Arc because it is always retroactive. Series that start out with a Story Arc or Myth Arc 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, Arc Welding itself can be plotted from the beginning, as The Reveal. In the absence of the confirmation provided by the Word of God, Arc Welding is a Subjective Trope.

Compare Canon Welding, which ties together different series, and Innocuously Important Episode, which retroactively ties a seemingly unrelated event into an existing Story Arc. See also Meta Origin, Patchwork Story.

Has nothing to do with welding metal using arcing electicity. For info on that, go here. For a minigame on that, go to Metroid Prime.

Anime and Manga

 * In Bleach, a large number of seemingly incidental minor villains and events which occured throughout the series are revealed to have been caused by the Big Bad in season 3.
 * This is made such a random Ass Pull by the fact that NOTHING COMES OUT OF IT.
 * Higurashi no Naku Koro Ni:
 * A minor Naruto example: A Filler episode revealed that Mizuki attempted to steal the scroll of kinjutsu on Orochimaru's orders. This revelation occurred around episode 160 or so, and Mizuki stole the scoll in episode 1.
 * A major Naruto example: Madara Uchiha is apparently largely responsible for/connected to the origins of Naruto (he unleashed the fox before it was sealed in Naruto), Sasuke (he helped Itachi with the Uchiha massacre), the entire Leaf Village (founder of one of the two clans it started with), and potentially many more elements of the story given his long tenure of Faking the Dead and being The Man Behind the Man
 * Double Subverted- Tobi was never the real Madara to begin with. He is still responsible for the Kyuubi attack and the Sasuke and Itachi stuff, and a whole lot of other things, but several others he took credit for were actually the product of the real Madara's schemes or actions, such as implanting the Rin'nengan in Nagato. Tobi was Madara's apprentice and hijacked his plans.
 * Not to mention that the 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 (what he is initially mentioned for), also is source of the Senju and the Uchiha clans, the guy who split the almighty ten-tailed beast into the more familiar one to nine-tailed beasts, creating the MOON on the whole process. Busy guy.
 * Every subplot in Excel Saga, which were all Played for Laughs, actually got tied together in the surprisingly serious finale.
 * In Sailor Moon DIC tried to do this to all the bad guys that appeared during the second season. Then later
 * Tenjho Tenge. Every bad thing that ever happened to anyone turned out to be the work of . Even the stuff that happened centuries ago.
 * Yu-Gi-Oh GX 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 Yu Yu Hakusho confirms that
 * Pokémon Special tends to do this. Both good guys and bad unexpectedly show up in later arcs with gambits ahoy, but FRLG and Emerald take the cake. FRLG was a result of all the previous arcs while Emerald was a direct consequence of FRLG.
 * Volumes 29 and 38 brought together and resolved so many plot lines from different arcs to the point that the HSQ was reached.
 * Happened quite a lot in Fist of the North Star. A few volumes after Kenshiro's Token Motivational Nemesis Shin was killed off, it is revealed that the reason why he turned on Kenshiro in the first place was because Ken's stepbrother Jagi (a later villain) persuaded him.
 * The Tatami Galaxy is built around this trope.
 * Pretty Cure All Stars DX 3 did the same thing the manga Sailor Moon did with Black Hole. Though since DX 3 is mostly non-canon to the series they're connected to, it kinda really doesn't count.

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 Myth Arc with issue #13 (the Year of the Bastard storyline), but starts Arc Welding 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 Don Rosa's The Life and Times of Scrooge McDuck series, which takes every random reference Carl Barks 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.
 * Geoff Johns has been doing this in the Green Lantern 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 has established whole rival corps for each (like yellow ring-wielder Sinestro starting his own Sinestro Corps of fear).
 * It all started with his revival of Hal Jordan. The Retcon that explained away Hal's villainy also accounted for the periods of self doubt the character had been through, the graying of his hair, the reason the Spectre chose him as a host, and the reason why Hal was tempted by the power he stole while Kyle wasn't.
 * When Marvel finally decided to fully reveal Wolverine's 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 Ultimate Spider-Man, several Batman Cold Opens 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.
 * Kurt Busiek's Avengers Forever story indulges in arc welding throughout, revealing that
 * One storyline of Archie Comics was the result of the events in previous stories: over the past couple of months, Jughead has experienced cranial injury from different objects (box of comic books, flamingo statue, typewriter, football, and a ball of hamburgers). Somehow this causes his body metabolism to go in reverse; wherein usually any food he eats is instantly digested (explaining why he stays so thin), all the foods he eats now turn into fatty tissue, resulting in Jughead gaining a lot of weight overnight.
 * Sonic the Hedgehog: During the "Enerjak Reborn" arc, Dr. Finitevus reveals how everything he's done since his introduction has been part of his grand plan to create a new Enerjak. It's also revealed that he was the (at the time) unnamed Echidna scientist who attempted to return Chaos Knuckles to normal by draining his power (the backfire of which is what rendered him albino).
 * Issue 224 reveals how several seemingly unrelated events during the Filler Arc following the defeat of the Iron Dominion actually tied into Eggman's plan to.
 * Issue 233: many of 's past actions are shown in a new light, as it's explained that he was working on a master plan to.
 * When the Destructix's backstories were revealed, a bit of this was applied to Sergeant Simian and Predator Hawk. They were shown to, in the past, have been members of the Gorilla Guerillas and the Battle Bird Armada (respectively), groups that were introduced after they were, but before the story arc that revealed their past memberships.

Film

 * Rare film example: in Scream 3, the killer,, reveals that he was responsible for the events of the first movie, being the one who inspired Billy to start killing. This would also make him indirectly responsible for the events of the second movie as well.

Literature

 * Sherlock Holmes. It was revealed, in the first of the two stories by Arthur Conan Doyle to directly feature Moriarty, that the master villain had been involved in several of Holmes' cases before, but had simply never been mentioned until then.
 * 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.
 * Doctor Who Expanded Universe:
 * The Virgin Missing Adventures novel ''Venusian Lullaby" takes a lot of the throwaway one-liners the Third Doctor used to make about his adventures on Venus (that the Venusians sing lullabies, play hopscotch, etc.) and makes a coherent alien race out of them (living about a billion years in the past, when Venus might have been habitable). They're radially symmetrical and move by hopping. Oddly, no mention is made of the oft-referenced "Venusian Aikido," though, nor how he might have learned it from such odd creatures.
 * The Virgin New Adventures novel Original Sin establishes that  was responsible for various advanced technologies that plagued the Third Doctor, and also created the glitterguns from "Revenge of the Cybermen".
 * Many Star Wars Expanded Universe sources tie in many of Palpatine's and especially Thrawn's actions as being to prepare the galaxy for the Yuuzhan Vong invasion. The degree to which this is accepted by fans varies- it's pretty much considered canon for Thrawn's motivation, but most see Palpatine as just using it as an excuse.
 * More recently, Jacen Solo's change of heart in Legacy of the Force is seemingly being tied to new Big Bad Abeloth in Fateofthe Jedi.
 * Another example is Cade Skywalker's unique abilities in Star Wars Legacy. His ability to sense vulnerable fractures has been tied to Mace Windu's sense for more metaphorical "shatterpoints", and his ability to heal serious injuries and death using the Dark Side is uncannily close to what Palpatine claimed Darth Plageius could do in Revenge Of The Sith. And considering that other sources hint that Plageius created Anakin, it may be connected to that too...
 * Back to the subject of Abeloth, the final Fate of the Jedi novel reveals that she's connected to the Son, the Daughter, and the Father, the living embodiments of the Force first seen in Star Wars the Clone Wars.
 * This sort of thing is quite common in Star Trek Novel Verse books:
 * The Star Trek the Next Generation novel Q & A has every Q-episode of TNG as being part of a long term plan to . Except "Q-Pid". That one he just did for the hell of it. Or possibly for the Foe Yay.
 * Star Trek Forged in Fire ties Sulu's captaincy of the Excelsior (as seen in The Undiscovered Country and explored further in Star Trek the Lost Era) to the Blood Oath plot from the eponymous episode of Star Trek Deep Space Nine. In other words, Sulu's story arc is now made part of the backstory of Curzon Dax and the Klingon trio of Kor, Kang and Koloth. The novel also ties in the Klingon Forehead arc, from Star Trek Enterprise and Star Trek Vanguard.
 * The Star Trek Typhon Pact novel Paths of Disharmony links the Star Trek Vanguard story arc to those of the Star Trek Deep Space Nine Relaunch and the post-Star Trek Destiny line, making it all relevant to the current events of the novel.
 * A rather pleasing example in the Star Trek Voyager Relaunch with the exploits of Kahless, retroactively linking the novel line's Klingon saga to the relaunch in interesting ways. In Star Trek a Time To, Kahless had replaced himself with a hologram (equipped with a mobile emitter) and wandered off to Cygnet IV, supposedly to "do whatever (he) felt like". It was also a test, allowing him to give his usual Hurricane of Aphorisms when the ruse was discovered. In the Star Trek Voyager Relaunch, though, it's revealed why he was on Cygnet IV specifically..
 * Any Christopher L. Bennett novel in the line (Ex Machina, The Buried Age, Watching The Clock) is like this. It sometimes borders on Continuity Lock Out, but never crosses the line.
 * The Q Continuum trilogy ties together three previously unconnected malevolent energy beings from the original series and movies along with the Galactic Barrier (yes, one of them is "God" from Star Trek V), and as an extension explains why proximity to the barrier resulted in the development of telekinetic powers and personality shift in the series pilot.
 * After Isaac Asimov's extensive Canon Welding of his Foundation, Empire and Robots series into one, the final volume of the Second Foundation Trilogy, Foundation's Triumphby David Brin was a fantastic example of Arc Welding together almost every single book in the series together even better than Asimov had done.

Live Action TV

 * One of the more explicitly identified examples of Arc Welding 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 Gambit Roulette.
 * A smaller version then happened in the next and final season: faced with Executive Meddling 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.
 * Star Trek: Deep Space Nine does excellent examples of Story Arcs and also of Arc Welding. 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 Myth Arc of the show.
 * Also, Executive Meddling resulted in the Klingons returning to their Kirk-era level of villainy just as the Dominion was planned to take center stage. The Powers That Be did the best they could, and made it fit at the end by making Changeling manipulation responsible.
 * The X Files 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 Arc Welding, was done in Veronica Mars, 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 The Reveal 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.
 * At the end of the first season of The Sopranos, Big Pussy has vanished. 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 The Mole, with him being a second one.
 * Doctor Who did a bit of Arc Welding near the end of its original run, when Ace's involvement with the Doctor between "Dragonfire" and "The Curse of Fenric" (eight multi-part stories over three seasons by that point) was revealed to be part of Fenric's Batman Gambit to trap the Doctor (moral of the story: don't try a Batman Gambit against a Chessmaster). It also provided a bit of disappointment for Classic Who fans when the series returned and "Bad Wolf" turned out not to be a returned Fenric.
 * In NuWho Series 5, the startling lack of historical commentary about a big freaking Cyberking stomping around Victorian London is retconned to have been another casualty of the cracks in time.
 * Season 4's finale revealed that the various plot threads of the season -- disappearing planets, Rose's brief appearances, "the bees disappearing", and references to Donna's uniqueness -- were all, one way or another, connected to Big Bad Davros' plan to create and detonate a Reality Bomb. This also tied in Dalek Caan's last appearance, which had actually been a season previous.
 * Stella used one frequent actor to set up an arc for the series.
 * Graem Bauer and the "Bluetooth Group" in Twenty Four. The cabal of influential businessmen in the fifth season ordering Christopher Henderson et al. to do their dirty jobs was initially planned to be a shadowy group whose true motives were never explained (and they had no connection whatsoever to Jack or his past). In the sixth season, the previously unnamed head of this group was revealed to "Graem Bauer", and it was explained that he was responsible for most of the government's shady activities going all the way back to the fourth season (when Walt Cummings tried to have Jack killed). Graem is then revealed to be The Man Behind the Man, as his (and Jack's) father Philip shows up and murders him in his very first appearance.
 * Season 7 does it again, revealing the season's Big Bad, Alan Wilson, to have been behind even the Bauers' involvement, and the ultimate authority over the Season 5 conspiracy, being head of a powerful group that had been manipulating events for some time and remained at large at the end of the season. Then the storyline disappeared without a trace.
 * A great many of the episodes of the '90s series of The Outer Limits were retroactively revealed to take place in the same continuity. Unfortunately for continuity, several of those episodes are entirely contradictory.
 * The BBC's recent Sherlock Holmes miniseries Sherlock had Moriarty pull this off again... albeit for an arc only three episodes long.
 * Power Rangers had two major examples of this:
 * Power Rangers in Space revealed that Dark Specter was behind everything that happened in the series to that point. Combining the narratives of all the previous seasons (Mighty Morphin, Zeo, Turbo, and In Space) into one greater arc.
 * In Power Rangers Ninja Storm it is revealed that each plot by the contractually genre blind Big Bad Lothor was actually part of a very Genre Savvy Batman Gambit.
 * Chuck welded arcs on top of arcs. About midway through the first season everything that happened up to that point was revealed to largely be the result of a CIA splinter group known as FULCRUM attempting to steal the Intersect. By late in Season 2, Chuck has begun to speculate that many one-shot villains earlier in the first season such as Laszlo were actually connected to the Intersect project and FULCRUM. The Season 2 finale reveals that Fulcrum was part of a larger organization called the Ring. The middle of Season 3 reveals that the Ring's direct involvement goes back much further, and that another one-shot villain from Season 2 was actually working for them. When Season 4 rolls around, you learn that yet another one-shot villain from Season 2 was actually working for new series big bad Volkoff Industries, and that the entire history of the Intersect project itself was directly connected to Volkoff himself. Finally in Season 5 you learn that rogue CIA agent Quinn was quite literally behind EVERYTHING--Fulcrum, the Ring, and Volkoff Industries--out of his desire to get revenge on Chuck for "stealing" the Intersect from him.
 * Kaizoku Sentai Gokaiger episode 40, which not only turns the Gokaigers' Early Bird Cameo (a Big Lipped Alligator Moment during the Goseiger/Shinkenger Reunion Show) a canon event, but also ties into the team's quest of Gotta Catch Them All.

Tabletop Games

 * Up until the Invasion block of Magic the Gathering, the storylines of the sets before were mostly World Building the plane of Dominaria. Then came Invasion, Planeshift, and Apocalypse, the Big Bad and his cronies from the previous stories are converging on Dominaria to overlay the artificial plane of Rath and give the Phyrexians a new world to ravage (class 3A Apocalypse How).
 * Also, the problems that were dealt with in the Time Spiral block were said to have done things which were major in other blocks, like making the Emperor insane, which convinced him to steal a baby kami, which resulted in the Kami War which made up the Kamigawa block.
 * And now they've welded the Phyrexia plot (The Nineties in a nutshell) with Mirrodin.
 * Hard to call that welding... according to word of god, they'd planned for Mirrodin to reintroduce Phyrexia from the start. They even included (extremely) subtle references to it in Mirrodin's block novels.
 * The Ravenloft D&D setting's creators retroactively Arc Welded its first six adventures together into the Grand Conjunction story arc, using a Vistani prophecy as solder.
 * Arguably, the D&D settings Planescape and Spelljammer, both of which were specifically created to allow the numerous other settings of D&D to mix with each other.

Video Games

 * Metal Gear Solid was mostly self-contained, despite being a sequel to the MSX games Metal Gear 1987 and Metal Gear 2 Solid Snake. Metal Gear Solid 2 Sons of Liberty introduced The Patriots, whose operatives were largely responsible for the events in the series. Metal Gear Solid 3 Snake Eater's place in the Patriots arc is  but it's missed by many until Metal Gear Solid 4 Guns of the Patriots spells it out because the Patriots as they exist by the time Sons of Liberty takes place are very, very different than the organization used to be.
 * In Guild Wars 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 Big Bad 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. Appropriately, 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.
 * "Evolution requires sacrifice." Spoken by of Mega Man X 8, and Serpent of Mega Man ZX. This refers to the evolution of the Reploids into something else...Granted, those two are in a future (and a world) that anything's possible with the Reploids (or technology in general). And then, in the Video Game Remake (and possibly Continuity Reboot) Maverick Hunter X, Sigma begins spreading these beliefs himself. If the reboot series wasn't axed, the developers could have elaborated on the concept even more.
 * Meanwhile, in Mega Man Zero, unlike the two series that preceded it, there is a different Big Bad for the first two games. Then, that novelty just had to be shot down by Zero 3, wherein that game's Big Bad, Dr. Weil, can be easily marked responsible for the events of the previous two, legibly making him the Big Bad for the entire Zero series.
 * Wily claims to have orchestrated Mega Man Battle Network 2 at the end of Battle Network 3, making the first three games one saga about Wily's plots to destroy the Internet (really) culminating with the Alpha plan in 3.
 * King's Quest: Dahlia, Hagatha, Mannanan, and Lolotte all appeared to be generic evil wizards and wicked witches. But then, we hear that the Big Bad in King's Quest V is the "brother" of Hagatha and Mannanan and looking for revenge. Then King's Quest VI has a single, damning letter alluding to the "Brotherhood of the Black Cloak," who address one another as "brother" and "sister." The letter implicates at least three of the series villains as members...and could possibly implicate all of them as members. Oh Crap. Fanon, especially the Fan Sequel and Fan Remake games will cheerfully run with this theory.
 * LA Noire:
 * Ace Attorney Investigations 1 and 2:
 * For almost 20 years, Sonic CD was a Gaiden Game with no set place in the series' timeline (Due to the lack of Tails or Super Sonic and the fact it was meant to be Sonic 2, most fans just assumed it was a prequel). However, as of the new remake for X-Box Live, Playstation Network, and iPhones, features were added suggesting (and Word of God asserted) that it actually occurred sometime before Sonic 4, and it will be specifically tied to Episode II.

Western Animation

 * Interviews with the creative team behind Justice League Unlimited and their remarks in various DVD audio commentaries reveal that they were several episodes into the production of Unlimited before they realized that they were working toward what became the "Cadmus Arc." The majority of the arc expanded upon and revolved around the events of "A Better World," an episode from season two of Justice League which had been written with no thought to an ongoing story. The ultimate reveal of the Big Bad, Brainiac, actually connected back to an episode of Superman: The Animated Series, which had been produced eight years and two TV series before the current series was even conceived. There were also throw-away lines and references that connected to as far back as "On Leather Wings," an episode of Batman: The Animated Series that was the very first entry in the DCAU.
 * Parodied in Clone High's final episode, where the 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
 * Most of the middle part of Gargoyles season 2 was already an arc, but then, during the two-part episode "The Gathering", 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 Story Arc 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 Story Arc finale.
 * Also, many unrelated aspects of the first half of the series were ultimately revealed as part of the Archmage's far-reaching Batman Gambit for world domination.
 * Transformers Animated had Ratchet's war flashbacks; in "Thrill of the Hunt" he flashes back to serving alongside Arcee, and in "A Bridge Too Close" he recalls having served in the war with Omega Supreme,, as an old friend. The third season reveals that
 * This happens with Codename Kids Next Door in its first season. It turns out that Father was behind a lot of the episodes.
 * Stroker and Hoop finale has them kidnapped along with Double Wide and put inside K.A.R.R over a canyon while being taunted by their kidnapper who gives them three guesses on who he is lest he drops them in the canyon. Through the whole episode the three call back recent episodes and go over suspects.

Webcomics

 * Its Walky/Roomies. The entire storyline consists of retconning arcs to weld them into bigger arcs reaching further into the past, culminating in a gigantic "conclusion" arc connecting every character that ever apeared in the comic into a huge overarching plot (whose final resolution endes the comic).