Sometimes, a story starts out with a small conflict, but then escalates to the point that the hero's party has to save the entire world. A Minor Crime Reveals Major Plot may be involved.
It doesn't matter what the party's original goals were (or those of its members); they're going to end up Saving the World from an unstoppable, inconceivable threat - and since You Can't Thwart Stage One, they will defeat said threat only when the world is right on the brink of doom.
A common manifestation is that despite the all-encompassing nature of the threat, no one else in the world will assist them, or even care. Forget about receiving a " saving-the-world discount" from shops, or a few armies to help them storm the Big Bad's lair.
Compare Grand Finale, where the scope of a serial work is expanded for the final (couple of) episode(s); Big Damn Movie, where a cinematic adaptation of a smaller-scale work has a "Save the World" Climax plotline; Minor Crime Reveals Major Plot (formerly Gonna Need A Bigger Warrant), which is about the investigation of a small crime leading to the discovery of a major crime/conspiracy; and Sequel Escalation.
A hero who finds himself drawn into this task may find his character archetype subject to Messiah Creep. If this plot keeps being recycled even after the world is saved once, you have The World Is Always Doomed.
As a Plot Twist trope, expect spoilers.
- Fullmetal Alchemist starts off with Ed and Al searching for the Philosopher's Stone to retrieve their old bodies. After the nature and goals of the Homunculi are revealed, their fight switches to stopping Father from absorbing the Gate of Truth and destroying the world.
- For much of Scrapped Princess, Shanon and Raquel's only goal is to protect their adopted sister, Pacifica, from the Church of Mauser. It isn't until they're captured by Princess Senes that they learn the truth about the church, Pacifica, and themselves, as well as what's really at stake. Shanon and Raquel then realize that they're protecting far more than their sister.
- Origin: Spirits of the Past starts with a Fish out of Water exploring a new world and ends with saving the World Tree.
- Naruto starts with the misadventures of a bratty outcast in search for recognition, and ends with saving the planet from an Assimilation Plot.
- Neon Genesis Evangelion plays with the trope: every fight has the fate of humanity hang in the balance, but it's all a plan by a few fractions of humanity to initiate the end of the world on their own terms. As their plans collide, the end of the world ends up in the hands of a mentally fractured teenager, with disastrous results.
- Code Geass starts being about a fallen prince's vendetta against his country of origin and a terrorist group struggling for their country's independence, and ends up being about saving the world from one Assimilation Plot and two nigh-omnicidal well-intentioned extremists in quick succession.
- Part 6 of Jojos Bizarre Adventure follows this. It starts as a prison drama about an inmate trying to reclaim her father's memories, then becomes a race to prevent a crazy priest from resetting the universe. They fail...somewhat. They aren't able to stop him from gaining a power up to do so. But his end goal to was reset the world without a Joestar line as to avenge Dio. Despite killing near all the main heroes. The one remaining ally manages to off him at the last possible moment. The world is reset but the Joestar line survives.
- Mahou Sensei Negima! begins with Negi's misadventures as a teacher of an all-girl junior high school class with the goal of finding his father. The final arc involves him and his Battle Harem saving not Earth, but another world entirely. Interestingly, saving the world is the end goal of the villains, they just have a different method of doing it that involves the sacrifice of a lot of the denizens of said world, who are just illusions.
- Kill la Kill starts with a girl who wants to discover the identity of her father's killer, and ends with her fighting her mother who is the evil CEO of a clothes company that has 100% of the global market; their clothes actually contain the spawn of an Eldritch Abomination (that said CEO worships) whose life cycle will consume and destroy the Earth.
- Little Witch Academia (2017) is from the same people that made Kill La Kill, and has a similar structure: the first half is mostly episodic adventures in an enclosed space (an academy in both cases), then the Big Bad is revealed in the second half and the threat rapidly escalates. In this case, Akko the witch goes from attending magic lessons to saving the world from being blown up by a literal magic missile, i.e. an ICBM possessed by dark energy as a result of one of the Big Bad's experiments gone haywire.
- Star Wars: Luke Skywalker is your average moisture farmer living in the dusty ass-end of nowhere when BAM! This random astromech droid starts playing back a message from a princess in need of saving from the Big Bad. Being the impressionable young man he is, Luke embarks on a mission to rescue the Damsel in Distress and maybe discover something about himself along the way. Simple, right? Wrong. Before Luke has time to say 'I've got a bad feeling about this', he's undertaking a pivotal role in the war to overthrow the evil Empire and confront his destiny to decide the final battle between Good and Evil.
- Constantine: John Constantine is hired to investigate a suicide. In the end, he tries to stop a conspiracy to bring the son of the Devil on Earth.
- TRON: Legacy is originally about rescuing the previous film's protagonist from imprisonment in virtual reality, and ends up being about saving the world from an invasion by a fascist AI.
- 2013 comedy The World's End, the last film of the "Three Flavors Cornetto" trilogy is about a group of old friends who reunite to re-enact a legendary pub crawl from their youth. At some point their pub crawl becomes a battle to save the human race.
- Ghostbusters (1984): What started as three disgraced ex-graduates with a plan for a Zany Scheme ended with four heroes staring down an ancient god of destruction...and winning.
- Kingsman: The Secret Service: Most of the movie is about the next Lancelot's training and Galahad investigating Valentine. Then Eggsy, Roxy and Merlin have to prevent Valentine's apocalypse.
- Babylon 5 starts out with rather minor border skirmishes between the Centauri and Narn, some diplomatic tensions among other races, and an Earth Government moving slowly in a xenophobic direction. When the Shadows reemerge into galactic affairs, they manipulate and cultivate these existing tensions into a galaxy-wide war among the younger races which escalates to an apocalyptic scale.
- Star Trek: Deep Space Nine starts with the Federation taking over an old starbase from the Cardassians who'd recently withdrawn from a long brutal occupation of the planet Bajor which the starbase orbits. Episodes involve the rebuilding of Bajor and its various growing pangs of independence (and bitterness over its recent past), and some exploration through a wormhole recently discovered near the station. But a great power lay on the other side of that wormhole, which soon puts the whole Alpha Quadrant in jeopardy in the large-scale Dominion War.
- Buffy the Vampire Slayer: The titular character starts out just trying to kill the local vampires, but quickly starts having to thwart the attempts of an old, powerful vampire, the Master, who is trying to get out of the magical cage he is trapped in. By the end of the season, she has to stop the Master from opening the Hellmouth, an act which would lead to a flood of demons entering the world and destroying all human life. The second season escalates the threat from "just" loss of all human life to loss of the very existence of the world.
- Supernatural: The show starts with two brothers hunting ghosts, werewolves and other supernatural creatures in the US while looking for their father. In the fifth season, they're trying to prevent the Biblical Apocalypse from occurring. In the Post Script Seasons that follow, the world has to be saved several more times.
- Bleak Expectations starts out as a straightforward (if often surreal) parody of Charles Dickens' life stories such as Great Expectations or David Copperfield. Thanks to Serial Escalation, by series two the antagonist has crossed the line from everyday villainy to cartoonish supervillainy, and in the Season Finale the hero has to save the world from an Alien Invasion.
- Persona 4 starts off with a group of friends discovering the (supernatural and thus unprovable) MO of a Serial Killer and resolving to catch him. If you manage to unlock the true ending it turns out to be a conspiracy by an Eldritch Abomination (disguised as a gas station attendant) to bring about The End of the World as We Know It.
- Persona 5 starts with a group of friends travelling into an alternate universe formed from the human subconscious to perform HeelFace Brainwashing on the wicked adults in their lives. As the game progresses, the steaks are raised as the heroes must save their country from being taken over by a politician weaponizing said alternate universe, before taking on a wicked god hoping to subject humanity to his whims. And just in time for Christmas!
- Final Fantasy IV begins with Cecil's demotion for questioning orders and being sent to deliver a package. This spirals out of control until you find yourself going to the Moon, fighting a Humongous Mecha that is going to annihilate all life on the planet, and then doing battle with the incarnation of all evil.
- Final Fantasy VI starts off with a mysterious girl named Terra working for the empire due to a form of mind control. She has no knowledge of who or what she is. When she gets knocked out in a mission, a treasure hunter named Locke quickly helps protect her. Before long she's helping The Resistance fighting against The Empire. The Empire itself poses a threat to the world, but the emperor himself would never go so far as to destroy it, which his seemingly comic-relief jester Kefka goes ahead and does just that. The world now in ruins, the heroes know that they at least have to stop Kefka from destroying all existence since he'd already gone that far.
- Final Fantasy VII starts with a resistance group known as Avalanche fighting against the Shinra Mega-Corp just to help improve the lives of the citizens that live in the slums. One thing turns to another and eventually they're fighting a psychotic Super Soldier who wants to destroy the world.
- Final Fantasy IX starts with a performance troupe putting on a play as a distraction while they kidnap the princess from her domineering mother of a queen. One thing leads to another and soon a weapons merchant is threatening the destruction of not one, but TWO different worlds. And then the personification of Death shows up to determine whether the universe itself is worthy of existence.
- Final Fantasy X-2: The game starts as a Lighter and Softer sequel, with Yuna and Rikku as Sphere-Hunters, Magical Girl-esque Transformation Sequences, a Goldfish Poop Gang as the primary antagonists and Fanservice ahoy. Then Vegnagun shows up halfway through Chapter Two and all of this is dropped in favour of stopping Shuyin before he can activate Vegnagun, resulting in an Earth-Shattering Kaboom - with Magical Girl-esque Transformation Sequences, a Goldfish Poop Gang as allies and Fanservice.
- Xeno Gears starts off with Fei just living his normal life in a quiet village, devoid of conflict. Then The Empire invades and razes the town to the ground, forcing him to defend himself in the heat of the moment. One thing leads to another and then he's fighting against a man-made God to save the world.
- Neverwinter Nights:
- The original campaign begins with the effort to find a cure for the plague ravaging Neverwinter. It ends with Neverwinter fighting a full-scale war, and you trying to prevent the entire Sword Coast from falling back under the dominion of a 30,000-year-old sarrukh queen.
- Hordes of the Underdark begins with drow raids on Waterdeep, and ends with a goddamn archdevil trying to take over Faerun.
- It's easy to forget these days that Mass Effect's Commander Shepard wasn't born killing Reapers. Minus out-of-character knowledge you start the first game expecting an apprenticeship of sorts under a turian Spectre named Nihlus. That isn't the case for long. And until the Reapers actually arrive in Mass Effect 3 the only folks who believe you and are willing to help are the ones who either know you personally (your squad and crew, mainly), and crazies like the Illusive Man.
- Particularly galling in Illusion of Gaia, where your goal is generally exploring ancient ruins and collecting mystic statues.
- Crono from Chrono Trigger starts his game by waking up and going to a local carnival. While there, he meets a pretty girl who gets sucked into a time warp in an accident involving a portal machine on display there. He follows to save her, and a few plot points later, he and some friends are questing all over the Earth through different time periods to save the world from annihilation at the hands of an incomprehensible Eldritch Abomination.
- Skies of Arcadia spends most of its time as a charming adventure revolving around exploration and piracy... until the final ten hours or so, when The Very Definitely Final Dungeon is raised from the depths of the planet, a devastating superweapon is unleashed, a country is leveled, and Heroic Sacrifices abound.
- Dragon Age: Origins plays with this trope. While technically you are saving the world by stopping the Blight (i.e. a vast horde of evil monsters led by a corrupted Dragon-God), the game's codex makes it quite clear that failure on your part will not actually lead to the end of the world. Blights reoccur every few centuries in Thedas, so people who dedicate their lives to stopping them have created a military organization, the Grey Wardens, just for that purpose. If you do not succeed, then one of the other members of your organization, which is thousands strong, would finish it in your place. By stopping the Blight, all you really do is keep the country that you live in from being destroyed before the other Wardens could act. Your victory simply means that the threat ended before the rest of the world noticed the problem.
- Dragon Age II averts this: Templars and mages will end up fighting one another, but how that comes about is up to Hawke, who can end up either a symbol of rebellion or an accessory to a war crime.
- Warcraft III starts with Thrall leaving the continent to find another one where his people can find a place to leave in peace, while Arthas investigates an epidemic. He then fights against a growing army of undead that threatens his kingdom. The climax has every faction of the world making a Last Stand against The Legions of Hell who want to destroy all life in the universe.
- Newer Resident Evil games not only underwent a Genre Shift, but also a thematic one: Older games were straight Survival Horror where the main focus was getting out alive, but 4, 5 and 6 all start with the heroes investigating an incident on a relatively small scale, only to instead uncover and foil a plot to either take over or destroy the world.
- Burning Rangers at first starts off as you playing as a futuristic fire fighting crew that tackles raging fires and rescue survivors. However, near the end of the game the Rangers discover an inferno satellite that is collecting space debris from across the galaxy that's on a crash course with Earth. The Rangers therefore quickly head into the ship's central core in an attempt to destroy the satellite and save the world.
- The main Pokémon games, at least since Gen III, barring most of the remakes. It always starts out as some kid leaving their small town on a journey To Be a Master, and escalates to them saving the world from the regional criminal organization who plans on Taking Over The World (or reshaping it) using the version's indicated legendary pokemon. Gen IV ups the ante, where the protagonist is forced to intervene in order to save all of existence.
- The Pokémon Mystery Dungeon games always begin with you helping your partner to chase their dreams, then eventually reveal you're a Chosen One who needs stop some manner of world-ending crisis.
- Luigi's Mansion: Dark Moon goes from Luigi having to prevent Ghosts from causing chaos in Evershade Valley to stopping the entire dimension from collapsing.
- In Ultimate Angler/StreetPass Fishing, for most of the game, your objective is to catch as many species of aquatic animals as you can, with the occasional mysterious specimen to give to a local marine biologist for study. The final thing to fish up, however, is a flying saucer belonging to an extraterrestrial who had crash-landed into the ocean and had been stuck down there. He was about to eliminate humanity out of frustration until you rescue him by fishing up his spaceship and said marine biologist fixes it so he can leave. Hence, by fishing it up, you inadvertently save the world.
- Rampage 2: Universal Tour starts with the monsters doing their usual business of destroying every major city in the world, but by the end, through their blind path of destruction, they have thwarted an alien plot to conquer the galaxy.
- In Atelier Sophie: The Alchemist of the Mysterious Book, Sophie spends most of the game trying to improve her alchemy in order to restore Plachta's memory and find the Cauldron of Knowledge. Once she does find the Cauldron, however, the game's Big Bad steals it and uses it to perform Ablation Alchemy, which increases his power at the cost of draining the world's resources, and Sophie must defeat him in order to save the world. And this is from a game whose tagline is "It's not always about saving the world"!
- In Jak and Daxter: The Precursor Legacy, the plot of the game begins with Jak and the recently transformed Daxter going on a journey to find Gol Acheron, the only sage who can change Daxter back. Midway through the game, however, it's revealed that Gol is actually the game's Big Bad who plans to unleash Dark Eco on the world to reshape the world to his desire, changing Jak and Daxter's objective to stopping him.
- The Order of the Stick: The first arc of the story concerns the Order carrying out a standard dungeon crawl in order to defeat a lich. Then the next arc reveals that within the dungeon was a gate to a prison dimension where a world-destroying Eldritch Abomination is kept, and the lich was planning to use the gate for nefarious ends. The rest of the story concerns the Order's efforts to keep the other gates from being used for evil, so that the abomination isn't set free to destroy the world.
- City Face parodies this. It starts off with a pigeon named City Face attempting and failing to woo an attractive female pigeon. A fairy shows up to help—and she tells him that the continued existence of the world depends on City Face successfully winning the female pigeon's heart.
- Worm starts out as a superhero story with primarily street-level conflicts. The last few arcs of the story, however, feature a threat that could wipe out humanity in not just one world, but every possible reality.
- The DuckTales (1987) Grand Finale, The Golden Goose, starts out with tussle between Scrooge and Glomgold over the cursed artifact (which can turn anything it's beak touches into gold, with the right magic word). The story escalates when the Golden Goose sets off the "golden death" - and Scrooge must return the goose to it's magic fountain before the entire world turns to gold! Just barely managing to do it.
- Justice League Unlimited ends as it began, with an alien invasion, this time by Darkseid and his forces. The scale was so massive that both heroes and villains had to team up to fight them. And of all people, it's Luthor who ends up saving the day by using the Anti-Life Equation to send both to the Source Wall, causing Darkseid's forces to retreat.
- Kim Possible ends with an alien invasion lead by Warhok and Warmonga, the latter of which whom had appeared earlier in the 4th season and having come back to enact revenge on Kim (who stopped her last time) and Drakken (who manipulated her into working for him). A threat that forces Kim, Ron, Drakken and Shego to work together to save the day.
- Mighty Max end with Skullmaster reforging the Crystal of Souls and reviving all the previously slain villains to hunt down Max for his cap to complete his plans. Despite trying to Summon Bigger Fish by teaming with the Lava Lord, Skullmaster is too powerful, defeats the Lava Lord, and ultimately gets the cap so he can enact a ceremony to reshape the world as he saw fit. It's only due to Max interrupting and contaminating the ceremony at the last minute that the world is saved...albeit at the expense of throwing Max back to the day he got the cap. But hey, not all bad, Virgil and Norman, who died during the finale, are alive again and the three still have their memories of the events of the previous timeline, meaning they now have an edge over Skullmaster.
- Scooby-Doo! Mystery Incorporated begin with the gang discovering a conspiracy going on in their town that dated back centuries and ends with them against a demonic entity that ends up freed and set to unleashed it's evil on the world. Scooby saves the day, but the timeline is changed since, without the entity's malicious influence, the people around them lead better lives. Good for them but rather bland for the gang since mystery solving was pretty much their trade. Least until they get a message from a professor who informs them the town may be clear of mysteries but there are others to solve beyond it. Time for a road trip!