The existence of the town, the human race, the universe, etc. comes down to the continued survival of one individual. For most of their lives, this person may not have been all that important in the grander scheme of things, but in this case at least, they are ultimately tied into the fate and destiny of their world. (Or at least enough people believe so to make a plot out of it, even if it later turns out that the character didn't really need to be protected.)
The goal, therefore, is to protect this person no matter what the cost. If they die, are gotten hold of by the wrong person, or are even just in the wrong place at the right time, then bad stuff will result. Basically, this is what happens when the Living MacGuffin meets the Butterfly of Doom, without necessarily having the fantastic, world-shaping elements that those plots usually require. The reason the character is important may be relatively mundane, emotional, and/or circumstantial in nature. They need not be a self-contained weapon, a Cosmic Keystone, an artefact disguised as a living being, The Chosen One, Messianic Archetype, or a Barrier Maiden; they may not even have any special abilities to speak of (though that depends on the genre of the story). In some cases, they may have a strong emotional connection and relevance to the cast, so their death or even injury would result in the cast going off the rails somehow.
The other characters may not even know exactly why they're protecting this person. They may just have to be in a certain place in a certain time, or they may just have enough influence on others or on their surroundings that eliminating them would cause the situation to become incredibly tedious at best, and apocalyptic at worst.
Compare with Living MacGuffin, Barrier Maiden and Apocalypse Maiden. When the hero's relationship with the person in question is the one that decides the world's fate, you get Sekaikei Genre. Sometimes subject to Prophecy Twist, in which case compare No Man of Woman Born.
- In Dragon Ball Z: During the Android Saga, Trunks, who comes from a Bad Future, goes back in time to give Goku the antidote to a heart virus from Planet Yadrat that he had originally died from to hopefully save the world from the Androids. Goku still ends up dying (though to a Heroic Sacrifice rather than succumbing to the virus), but the world is saved this time.
- One of the later arcs in Naruto is about a war being fought to protect both Naruto and Killer Bee from from having their tail-beast extracted by the Big Bad, Tobi.
- King of Thorn: Done subversively and surreptitiously in the manga — where everyone rallies to protect Kasumi, (who is in fact, Medusa) throughout the entire manga series. Their actions are mainly due to a directive imprinted in their minds by the Big Bad: "protect the small Japanese girl with your life". Most of them weren't even aware they were doing it. Of course, the bad guy wanted to reshape the entire world, not save it, so...
- Steins;Gate: Okabe saving Makise Kurisu is one part of preventing World War III in the not so distant future.
- Puella Magi Madoka Magica: If Kyubey gets to make a contract with the title character, she will quickly become an unstoppable monster that will raze the Earth to the ground. "Saving" her is, ironically, more a matter of stopping her from trying to sacrifice herself for the greater good. Adding further irony, this situation came about because of Homura's many failed attempts to save her via "Groundhog Day" Loop, giving her an impossible amount of "grief" energy. In the end, Madoka gets out of her predicament and saves the world herself through Loophole Abuse, but in The Movie, it's inverted, as Homura rips both Madoka and the entire universe apart to "save" her retroactively, and against her will.
- Nodwick: Nodwick learns, in a variety of ways that he simply must exist for the world to continue relatively peacefully. In the Baphuma'al series, his companions would have succumbed to various artifacts or neuroses had he permanently died at some point.
- Not to mention if Piffany didn't exist, Nodwick would be dead (since nobody was there to revive him each time he was killed) and Yeager and Artax were corrupted and ended up as Baphuma'al's Co-Dragons.
- X-Men: In the classic "Days of Future Past" storyline, Kate Pryde must prevent the assassination of anti-mutant senator Robert Kelly to prevent the Sentinels from conquering America and destroying all meta-humans.
- Fantasy of Utter Ridiculousness: Yukari Yakumo has been doing this in regards to Megas, believing that the mech has to survive long enough to turn back the Glorft invasion in the future if Gensokyo's existence is to be guaranteed.
- The Terminator: kill Sarah Connor, and her son will never grow up to lead the human resistance and defeat Skynet.
- Terminator 2: Judgment Day: Kill John himself, and he can't lead the Resistance.
- Again in Terminator Salvation: the machines try to kill Kyle Reese so that he will not go back in time and become John Connor's father, and the plot revolves around saving him.
- In Bill & Ted's Excellent Adventure, apparently the reason Bill and Ted are given the time machine.
- Willow: Elora Danaan is destined to overthrow evil queen Bavmorda, the irony being it's her protectors who defeat Bavmorda because she was trying to kill the baby in the first place.
- Lampshaded in The Pacifier, where the lead character is incredulous to learn that "protecting these kids is a matter of national security". Vin Diesel's vacant "WTF" stare at the news is priceless.
- The Purge: Election Year: Senator Roan plans to abolish The Purge if she's elected President, and is a very popular candidate because of that. As a result, the NFFA plan to have her assassinated during The Purge that year. So, Leo now has to keep her alive until the Purge is over.
- X-Men Film Series
- X-Men: Days of Future Past: Wolverine time travels from 2023 to 1973 to team up with the younger Xavier and Beast in order to stop Mystique from assassinating Dr. Bolivar Trask. If Trask is murdered, it will set off a chain of events which will lead to the extinction of mutantkind.
- X-Men: Apocalypse could've been titled Save Charles Xavier, Save the World. If Apocalypse succeeds in possessing Professor X's body, the former gains the latter's telepathy. Apocalypse can then combine it with his Super Empowering skill to put the entire planet under Mass Hypnosis, kill off anyone he deems to be weak with a thought, and everyone else becomes his mindless slave. This will cause The End of the World as We Know It.
- In the Star Trek: The Next Generation Novel Metamorphosis, after Data is granted his wish to become human, he is shown that his unique android abilities were the one thing which could prevent an all out war. Should he choose to stay human millions of people will die simply from his lack of ability to help them.
- An Isaac Asimov short story had this as a plot element. A man is pampered and kept away from all harm. The reason? It has been predicted by the Multivac computer that when he dies, the world ends.
- The apocalyptic tone of The Wheel of Time has made it clear that if The Chosen One, Rand al'Thor, does not fight at the Final Battle, Rocks Fall Everybody Dies. (Of course, that may still happen even if he wins, so there are no shortage of people who think it's "Kill This Person, Save The World" instead.)
- Insomnia by Stephen King culminates in the main characters racing against time to save the life of a young boy named Patrick...whom they've never met, never actually meet during the course of the novel, and whose importance is never quite fully explained in the book either. All they know is that the fate of the universe depends on them saving this one boy. The boy later grows up and becomes a pivotal character in The Dark Tower.
- In A Wind in the Door, Charles Wallace is the only person in any significant, physical danger. But his survival at this point in time is key to the continued existence of the universe as a whole. It's explained that this is the case because he has an important role to play in the future, and if he dies now the bad guys (who want to unmake EVERYTHING) will have a guaranteed win shortly down the line.
- Katniss Everdeen as the Mockingjay of the rebellion, starting at the end of Catching Fire. "While you live, the revolution lives."
- By extension it applies to Peeta as well. During the Quarter Quell saving Peeta was a high priority for a lot of the tributes, even at the cost of their own lives. In Mockingjay we find out that President Coin wanted Peeta saved above Katniss, feeling he was more valuable to the cause (and less likely to become a problem to her).
- In All Our Yesterdays, this is inverted. To save the world, James must be killed.
- Heroes. A whole season's Arc Words are "Save the cheerleader, save the world.", referring to Claire as being fundamentally important to the bad guy's plan for the reason that her particular powers (Wolverine-like healing) are the one thing he needs to fulfill his plot. It turns out that the reason for this is not so that Sylar doesn't gain her powers - by saving her, Peter becomes the "exploding man" who will destroy New York. Rather, we find out that because she's Peter's niece, she manages to get through to her biological father, Nathan, who proceeds to make a Heroic Sacrifice by using his ability of Flight to remove an unstable Peter from the range of New York to detonate harmlessly in the skies above. Both of them turn out alright - Peter has Claire's Healing Factor, and Adam (who has the same power) later uses his blood to heal a badly-irradiated and scarred Nathan.
- As mentioned above, John Connor in Terminator: The Sarah Connor Chronicles.
- Inverted in the Star Trek classic, "City on the Edge of Forever": Save the person, doom the world.
- Doctor Who has an entire episode revolve around this. In "Turn Left", it is revealed that Donna meeting the Doctor saved the multiverse, as without her he would have died (along with several other companions who filled his role in various episodes) and nobody would have been left to stop Davros' Reality Bomb.
- When the Doctor was dying, fading from existence because of the Great Intelligence, all the planets, solar systems, etc in the universe started dying/disappeared/becoming destroyed. Clara jumped into the Doctor's timestream to save his life, and scattered 'echoes' of herself across time, saving him in every dangerous situation he's ever been in. Effectively saving the entire universe.
- Farscape had an unusual example where this trope applied to the hero from his (sometimes) antagonist's point of view. Due to Crichton wormhole knowledge, Scorpius believed that he had to protect him for the coming war, to save the Peacekeepers and by extension the galaxy. Eventually, it paid off.
- Baby Eve from Being Human is an inversion of this. All of the good guys protect her, because she's the War Child, who prophecy says will kill all of the vampires, which means all of the vampires are trying to kill her. Then, it turns out that she has to die to bring about the death of all vampires, meaning that the protagonists who want to keep her alive are inadvertently screwing the world over. It's never really explained how Eve living would cause the vampires to win, but it's theorized that as long as she was alive, humans would stay lazy and expect her to be the one to save them.
- Castle, of all shows used this in the 4th season—Castle, Beckett, and the CIA are racing against time to find the 'linchpin' a mysterious organization is targeting to trigger World War III and bring the US to its knees. It turns out to be the daughter of an influential Chinese businessman; her assassination is projected to lead to the businessman turning his influence against the US, leading to a falling out with China which leaves the US in a crippling debt crisis. This question is Lampshaded at the end of the episode.
Castle: Do you think we actually saved the world?
Beckett: I think that... we saved a little girl's life. And that's enough for me.
- Hera, the half-human, half-Cylon child on Battlestar Galactica (2003). Throughout the series, it's implied that she is the future of both races. It's revealed in the finale that all modern humans are descended from her.
- This is the premise of the first-person shooter Darkest of Days. The player is kidnapped by a mysterious organization and must travel through time to various historical battles and ensure that key individuals survive as part of an unclear and ongoing conflict between said organization and another group.
- This is subverted in the end of the game: it is revealed that said individuals' descendants were responsible for developing technology that fell in the wrong hands and was used to create a bio-weapon that devastated the world's population. The group opposing you is actually a future incarnation of your organization that decided to forgo their "Don't change history rule" to prevent the disaster.
- Subverted in Dissidia: Final Fantasy: After Cosmos, guide to the protagonists, dies, the heroes still manage to gather up some Heroic Resolve and continue on with their mission.
- Done in Starcraft II, where it's Kerrigan. From Zeratul's and the Overmind's visions of the future, the Dark Voice will take control of the Zerg Swarm, and unlike Kerrigan has no problem using it to destroy everything, resulting in the extinction of the Terrans and Protoss, then killing the Zerg off once they're no longer necessary. If Kerrigan were to survive to that point, the swarm would still have a leader, thus preventing the Dark Voice from getting his army.
- You, in Baldur's Gate and its sequel in what is probably the most extreme that We Cannot Go On Without You has ever been taken to.
- Not all that overt, but in Devil Survivor, the entire possibility of saving humanity hinges on the suicidally-depressed singer Haru. Fail to save them, and you're locked out of the vast majority of the Multiple Endings, and in all likelihood stuck on the worst path of the lot.
- If the above person dies, then all of humanity's hopes rest on Keisuke's shoulders, as his demise causes a broken Atsuro to abandon his plan to solve the lockdown crisis, leaving you completely stuck on the option that causes the Downer Ending.
- Chloe "Karma" Lynch serves as one of these in Call of Duty: Black Ops II. As the creator of the Celerium worm that Menendez plans to use to crack the US defence network, she is the only one in the world who can stop it once it's been unleashed. As such, she is sought out by both sides, and her survival is paramount to getting the best ending.
- Justice League Unlimited: It's predicted that the death of the Flash will be a trigger for Armageddon since he's the Plucky Comic Relief keeping them on the rails. (In a Mirror Universe, his death instigated the rise of the Justice Lords). Flash lampshades this in "The Doomsday Sanction", to his own smugness and everyone else's annoyance. Only the Question is convinced it'll happen and attempts to kill Luthor himself to save Superman from it. This ends up making everything go From Bad to Worse when Luthor reveals his Evil Plan. Flash does end up defeating Brainthor, so who knows...
- Fry, from Futurama has saved the world and universe twice due to being the only one without a Delta brainwave (due to past-nastifacation with his grandmother), so Nibbler makes sure he gets frozen.
- In Young Justice, this is Impulse's goal. To avert the conquest of Earth in his time, he went back in time to save at least two people. He managed to save his grandfather Barry Allen (Flash) but that's not enough. The one he really needs to save is Jaime Reyes, who would otherwise have his Scarab rebooted and become the vanguard of the Reach's invasion force. Impulse's mission is to prevent this...one way or another.
- In "The End: Part One" of Teen Titans, the Titans use this sort of logic when they decide to protect Raven to try to prevent Trigon from turning her into a portal and coming to earth. It doesn't work. Otherwise it wouldn't be called The End Part One.