History InnocuouslyImportantEpisode / WesternAnimation

** "[[Recap/AmphibiaS1E37ChildrenOfTheSpore Children of the Spore]]" introduces a mushroom with mind-control spores. While initially playing like a random MonsterOfTheWeek, the mind-control enzyme is used by [[spoiler: King Andrias]] to control Amphibia's wild life as a supplemental attack force for his robot army. The original mushroom, which is sentient, later pulls a HeelFaceTurn to help the heroes.

** In Season Two's episode of TLA, "[[Recap/AvatarTheLastAirbenderTheLibrary The Library]]", the place would play a role in revealing crucial information about the Fire Nation's weakness. It would later appear in the Season Two episode of ''Korra'', "[[Recap/TheLegendOfKorraS2E10ANewSpiritualAge A New Spiritual Age]]", to reveal how to prevent BigBad Vaatu's release from the Spirit World.

[[Recap/StarWarsTheCloneWarsS1E5Rookies "Rookies"]] sets up the Domino Squad subplot that weaves throughout The Clone Wars. [[spoiler: Fives' subplot will play a role that affects the series finale, and allows Ahsoka to escape and play a role in ''Rebels'' and ''The Mandalorian.'', while Echo plays a role in the show ''The Bad Batch.'']]
* [[Recap/StarWarsTheCloneWarsS2E4SenateSpy "Senate Spy"]] may initially seem like padding, but it ends up kicking off a major arc about the second invasion of Geonosis, and ultimately leading to the first appearance of zombies on the show. Additionally, Rush Clovis, Padme's UnluckyChildhoodFriend, makes his first appearance betraying her in this episode. It seems like a minor issue in this episode, but come Season 6, the love triangle between him, Padme, and Anakin results in Anakin taking a step towards the dark side and allows Palpatine to assert more authoritarian control over the Republic.

[[Recap/StarWarsTheCloneWarsS1E5Rookies "Rookies" sets up the Domino Squad subplot that weaves throughout The Clone Wars. [[spoiler: Fives' subplot will play a role that affects the series finale, and allows Ahsoka to escape and play a role in ''Rebels'' and ''The Mandalorian.'', while Echo plays a role in the show ''The Bad Batch.'']]

* ''WesternAnimation/StarWarsTheCloneWars'': [[Recap/StarWarsTheCloneWarsS2E4SenateSpy "Senate Spy"]] may initially seem like padding, but it ends up kicking off a major arc about the second invasion of Geonosis, and ultimately leading to the first appearance of zombies on the show. Additionally, Rush Clovis, Padme's UnluckyChildhoodFriend, makes his first appearance betraying her in this episode. It seems like a minor issue in this episode, but come Season 6, the love triangle between him, Padme, and Anakin results in Anakin taking a step towards the dark side and allows Palpatine to assert more authoritarian control over the Republic.

* ''{{Franchise/Transformers}}:
** An episode near the end of the first season of WesternAnimation/BeastWars'' entitled "Before the Storm", which sets up the first season finale and a huge chunk of the second and third season subplots as well.

* ''{{Franchise/Transformers}}: WesternAnimation/BeastWars''
** An episode near the end of the first season entitled "Before the Storm", which sets up the first season finale and a huge chunk of the second and third season subplots as well.

* ''WesternAnimation/CodenameKidsNextDoor'' started its [[GrowingTheBeard beard-growing]] with what just seemed like another of its early, one-off episodic stints, namely episode [[Recap/CodenameKidsNextDoorS1E9AOperationCABLETV Operation: C.A.B.L.E.-T.V.]] where a baby network executive convinces the KND to let him use their satellite network, which turned out to be a plot to use his age-changing ray on the whole planet. The kids defeat the baby, but in TheStinger his age-changing device ends up in the hands of the Delightful Children...

** The Season One episode [[Recap/TheOwlHouseS1E8OnceUponASwap "Once Upon a Swap"]] is usually derided for its FreakyFridayFlip plot that is resolved easily and is never brought up again. After the GrandFinale, however, many fans notices Luz's appearance in this episode when she was swapped with King bore an eerie resemblance to [[spoiler: Luz' [[SuperEmpowering Titan-Empowered]] SuperMode from the finale]]. Some fans were irate over the newfound relevance of their least favourite episode.

** The Season 2 episode [[Recap/MyLittlePonyFriendshipIsMagicS2E20ItsAboutTime "It's About Time"]] is a typical slice-of-life episode with Twilight learning not to worry so much about the future after spending the week trying to prevent upcoming disasters. At the same time, it sets up two huge season-finale crises:

** "[[Recap/JusticeLeagueS2E11And12ABetterWorld A Better World]]", in which an alternate universe version of the Justice League assassinates President Luthor and seizes control of the government, becomes the basis for the conflict in the first two seasons of ''Justice League Unlimited''. The two-part finale of ''WesternAnimation/SupermanTheAnimatedSeries'' (which aired ''four years before'') also played a big part in this. Lampshaded thoroughly in the commentary tracks for ''Justice League'', along with a flashback to an even [[Recap/SupermanTheAnimatedSeriesS2E14GhostInTheMachine earlier episode of Superman.
** The season two opener "[[Recap/JusticeLeagueS2E1And2Twilight Twilight]]" seems like a one-off that concludes Darkseids' story in the DCAU until the two-part series finale, when [[spoiler:Luthor, in an attempt to revive Brainiac, brings back Darkseid with Brainiac enhancements.

* ''WesternAnimation/TheVentureBros'' takes this to ''an art form''. Seemingly trivial details and bits of dialogue have a nasty habit of becoming the fulcrums to entire episodes, up to several seasons later. Case in point: In the Season 2 episode "¡Viva los Muertos!" Dr. Venture educates his newly-animated Venturestein with a series of videos depicting a Central American sweatshop. Three seasons later, in the episode "Venture Libre", we learn that Venturestein became rebellious because ''he recognized one of the boys in the videos'' when he was sent to quell a revolution.

* The ''WesternAnimation/YoungJustice2010'' episode "Denial" initially looks like a straightforward VillainOfTheWeek outing. Ultimately, though it turns out that not only was aforementioned villain actually a member of [[OmniscientCouncilOfVagueness the Light]], the BigBad group of the whole series, but he was the ''first'' member of the Light to appear in person. Adding to that, the episode launched the ongoing Dr. Fate subplot that would turn up again several more times during the season, hinted towards Red Tornado's ties to the Justice Society, and laid the groundwork for the Kid Flash/Artemis relationship as well.

* ''WesternAnimation/CodenameKidsNextDoor'' started its [[GrowingTheBeard beard-growing]] with what just seemed like another of its early, one-off episodic stints, namely the episode where a baby network executive convinces the KND to let him use their satellite network, which turned out to be a plot to use his age-changing ray on the whole planet. The kids defeat the baby, but in TheStinger his age-changing device ends up in the hands of the Delightful Children...

* ''WesternAnimation/GravityFalls'' has some tremendous hints to its big mysteries hidden in episodes that otherwise aren't massively relevant to the plot. Season one's "Carpet Diem" is a FreakyFridayFlip episode, but includes some of the biggest hints to one of the show's mysteries. In the episode, we have two short moments where [[spoiler:Stan discovers an old pair of glasses slightly different than his own, and is later seen sadly holding them]]. Fifteen episodes later, we have [[spoiler:the Author's identity revealed to be Stan's brother]].

** In the first season, episode seven initially reads as an aside largely irrelevant to the broader narrative of the season, with the only meaningful advancement of the plot being that [[spoiler:Tulip learns her number has gone down at the end]]; otherwise it's largely a throwaway side story that introduces Tulip's reflection as a separate character in an apparent one-off. Come season two, however, Mirror Tulip and the pair of villains of the week for that episode are the primary drivers of the season's plot.
** In the fourth season, episode six has the Steward appear to return to Ryan and Min-Gi their belongings, and from it we hear [[spoiler:Amelia]]'s voice tell the Conductor he had done well to listen their suggestion. Episode nine makes clear that was the moment [[spoiler:Amelia's takeover of the Infinity Train]] had started.

** ''Queen of the Shadowkhan'' has Jade getting a tattoo from the symbol in a book, where she ends up possessed by it and can now rule the Shadowkhan ninjas. This hints at the fact that Shendu stole the Shadowkhan from somebody else, and the symbol is later revealed as the mark of Tarakudo, the BigBad of Season 4.
** ''The Chosen One'' has Tohru chosen as the prophesized master of a group of Monks, where a dark chi wizard ends up trying to hunt him to prevent said prophecy. Said wizard is Daolon Wong, who ends up becoming the main villain when he returns in Season 3.
** ''The Good Guys'' features Finn, Ratso and Chow giving up on evil, but later returning into it as they attempt to steal a magic opal. This same episode features the Ice Crew, who act as more competent versions of the Enforcers. They later return in Season 5 as they replace the Enforcers as the minions in the final season.
** ''J2 - Rise of the Dragons'' has a story where Jade sees two strangers from the future arrive, one being her future self, and the other being Drago, Shendu's son, who later returns as the final villain of the last season.

** "A Better World", in which an alternate universe version of the Justice League assassinates President Luthor and seizes control of the government, becomes the basis for the conflict in the first two seasons of ''Justice League Unlimited''. The two-part finale of ''WesternAnimation/SupermanTheAnimatedSeries'' (which aired ''four years before'') also played a big part in this. Lampshaded thoroughly in the commentary tracks for ''Justice League'', along with a flashback to an even [[Recap/SupermanTheAnimatedSeriesS2E14GhostInTheMachine earlier episode of Superman.
** The season two opener "Twilight (of the Gods)" seems like a one-off that concludes Darkseids' story in the DCAU until the two-part series finale, when [[spoiler:Luthor, in an attempt to revive Brainiac, brings back Darkseid with Brainiac enhancements.
** In ''WesternAnimation/SupermanTheAnimatedSeries'' Braniacs' second appearance had him kidnap Luthor to help rebuild his body after his first appearance, and Brainiac [[YouHaveOutlivedYourUsefulness had blast Luthor away when he had finished enough]], only to later be defeated by Superman. In the climax of the JL Cadmus Arc, it's revealed that Brainiac had infected Luthor with nanomachines through that blast, [[FridgeLogic explaining that's the only reason Luthor survived]], and had been influencing Luthors' actions for years and slowly rebuilding his consciousness within him. The STAS episode aired in 1997 while this reveal was in 2005, the creators admitted that was a plot thread they deliberately left but didn't expect to pick it up as late as they did.

** "A Better World", in which an alternate universe version of the Justice League assassinates President Luthor and seizes control of the government, becomes the basis for the conflict in the first two seasons of ''Justice League Unlimited''. The two-part finale of ''WesternAnimation/SupermanTheAnimatedSeries'' (which aired ''four years before'') also played a big part in this. Lampshaded thoroughly in the commentary tracks for ''Justice League'', along with a flashback to an even [[Recap/SupermanTheAnimatedSeriesS2E14GhostInTheMachine earlier episode of Superman]].
** "Twilight (of the Gods)" seems like a one-off until the two-part series finale, when [[spoiler:Luthor, in an attempt to revive Brainiac, brings back Darkseid with Brainiac enhancements]].

** Season 4 "[[Recap/TheAmazingWorldOfGumballS4E23TheSignal The Signal]]" has Gumball and Darwin being gone through mysterious disruptions that threatens their friendship and then their lives like a TV losing signal from a satellite. Then Gumball and Darwin starts to realize the satellite signal disruptions also affects THEIR world, they start to question the reality of this world until the episode forcibly ends with a EverybodyLaughsEnding. Of course this would be a one-time gag episode [[spoiler:until Season 4 finale "[[Recap/TheAmazingWorldOfGumballS4E40TheDisaster The Disaster]]" when Rob flat out tells Gumball that the world they live in isn't real and then "[[Recap/TheAmazingWorldOfGumballS5E1TheRerun The Re-Run]]" has Gumball try to use the Universal Remote and accidentally changes the channel to Elmore News which brings this instance again.]]

* The ''WesternAnimation/ElenaOfAvalor'' episode "Snow Place Like Home" is a typical Christmas episode that reveals Esteban's Freudian excuse, namely [[spoiler:how his parents died in a storm at sea]]. The last minute of the episode has a scene of Victor and Carla [[spoiler:meeting up with his wife/her mother]] , and it's definitely a doozy.
* The events in ''WesternAnimation/FamilyGuy'' episode "The Cleveland-Loretta Quagmire" would haunt Cleveland for years to come, such as in "Love Blactually," and eventually led to the existence of ''WesternAnimation/TheClevelandShow''.

** "Love's Labours Lost in Space" introduces the RidiculouslyCuteCritter Nibbler, who becomes Leela's pet. Much, ''much'' later, Nibbler would turn out to be key to the overarching plot of the series, eventually being revealed to be [[spoiler:the one who put Fry in cryogenic stasis and got him sent to the future]].
** "I Second That Emotion" features the first mention of the mutants who live in the sewers of New New York, eventually setting up TheReveal about [[spoiler:Leela's true origins]]. It even features a background cameo from two unnamed one-eyed mutants who would eventually be revealed to be [[spoiler:Leela's parents]]. It's also the first episode that hints that Nibbler is more than he appears to be, with a brief line from Fry serving as {{Foreshadowing}} that [[spoiler:[[Really700YearsOld he's actually hundreds of years old]]]].
** "Roswell That Ends Well" had Fry accidentally sleep with his own grandmother and [[MyOwnGrampa become his own grandfather]]. Obviously a ridiculous predicament, but it was not presented as any more important than the other ridiculous predicaments that have occurred throughout the series. However, "The Why of Fry" established that this is a major aspect of Fry being TheChosenOne, as this paradox left him without a "delta brain wave" (something he was established to lack in "The Day the Earth Stood Stupid" but was not explained at the time) and thus immune to mind control. This immunity, in turn, became a plot point in the fourth movie.

* ''WesternAnimation/CodeLyoko'': "Tip-Top Shape" initially appears to be a filler episode in which Yolanda is possessed by the evil AI XANA and Jeremy has a ZanyScheme to stop her by activating a tower to enhance Odd's abilities. However, it is important for two reasons. First, it is Jeremy's first time activating a tower, which ends up becoming essential to decode Franz Hopper's diary later in the season and becomes even more important in season 4. Second, Sissi becomes highly suspicious of the Lyoko Warriors and tries to learn their secrets, making her a more serious obstacle later on.

* ''WesternAnimation/DannyPhantom'' episode "The Million Dollar Ghost" has Danny fighting several expies of supernatural hunters such as Franchise/ScoobyDoo and dealing with the self esteem issues [[NiceJobBreakingItHero he accidentally gives his dad.]] However not only does this introduce [[TheMenInBlack the "Guys in White"]] government ghost hunters who serve as recurring antagonists from this point on, but the MacGuffin Vlad gains at they end is used to start the "Reign Storm" story Arc.

* The ''WesternAnimation/DuckTales2017'' season 3 episode "The First Adventure" is a WholeEpisodeFlashback about the founding of F.O.W.L. and how Scrooge first started adventuring with Donald and Della, with both groups going after the Papyrus of Binding, a reality-manipulating object that Scrooge's keeps away from F.O.W.L. by writing that it can only be reclaimed by his true heir, a means of keeping the villains from killing his nephew and niece and keeping the artifact out of their hand. In the season finale "The Last Adventure", the Papyrus of Binding is the final instrument needed in F.O.W.L.'s plot against Scrooge, [[spoiler:and to ensure they have an heir to reclaim it, they made a clone of him to become his true hair: Webby Vanderquack]].
* The ''WesternAnimation/EdEddNEddy'' episode "Ed, Pass It On" (from 2002) is about Eddy lying that his elusive older brother is returning to the cul-de-sac in an attempt to gain respect. When he supposedly does arrive [[spoiler:(it's actually Sarah and Jimmy in disguise)]], Eddy reacts with ''absolute fear''. Seven years later, the GrandFinale [[TheMovie Movie]] reveals that [[spoiler:Eddy's Brother is actually a sadistic bully who tortures Eddy for fun and all the stuff Eddy's been saying about him all these years were all lies so he can get respect from the other kids]].

** "Family Legacy" starts out as one regular adventure where Cricket and Tilly have to search for a family treasure that could save their home from being demolished and replaced with a yoga studio; it gets even darker once HistoryRepeats with the MultiPartEpisode "Chipwrecked"/"Chipocalypse Now".
** Taken up to eleven with "Reckoning Ball" after the shutdown of Big Coffee. Gloria says it's Cricket's fault for such; why? Because Cricket was the first one to believe Chip changed after taking advice from Bill, and the first to sign the forgiveness contract in reply. If he didn't believe Chip in the first place, Chip wouldn't have become the CEO and Big Coffee wouldn't have been closed! [[ItsAllMyFault Cricket soon begins to regret himself for this.]]

** In Season Two's episode of TLA, "[[Recap/AvatarTheLastAirbenderTheLibrary The Library]]", the place would play a role in revealing crucial information about the Fire Nation's weakness. It would later appear in the Season Two episode of ''Korra'', "A New Spiritual Age", to reveal how to prevent BigBad Vaatu's release from the Spirit World.

* ''WesternAnimation/TheAvengersEarthsMightiestHeroes'' episode "Alone Against AIM" seems rudimentary at first, boasting a simple storyline in which Iron Man and some of his companions prevent AIM from stealing important Stark Industries data and armor. It does, however, contain at least two ties to the season's main arc. First, Iron Man introduces a new suit of armor, which he will continue to use up through the series finale. Secondly, Captain America (actually a Skrull in disguise) reveals to the other heroes during the aftermath that he managed to keep the data out of AIM's hands, but [[spoiler:the Skrulls later use this information to implant a crippling virus into Iron Man's suit]].