Maybe you're Trapped in Another World, fighting monsters and saving people so you can find your way home... and then you do. Or maybe you've gone to another world after adventuring around your home, and found something a little too familiar about this place. Maybe you even go between worlds on a regular basis, and things are getting serious on the other side.
Either way, you discover the two worlds you've been flitting about in have a strange connection. It doesn't matter what it is, but it let you into the other world, and it can let something else out into yours. Generally it's monsters roaming the streets, but there can be other effects. If the more mystical of the worlds is based in a computer realm, expect a deadly effect on your hometown hospitals and roads.
The other world's still in danger, but now so is yours, and the Muggles certainly can't do anything about it. What do you do? You grab your adventuring party and a couple of friends from your homeworld, and you go out to Save Both Worlds.
Contrast with Up the Real Rabbit Hole, where both worlds are in jeopardy but the heroes don't see the one that isn't their own as real enough to merit saving. May overlap with The Call Knows Where You Live.
- Digimon Tamers would've just been "save Earth", but the Big Bad was directly tied into both worlds, and could only be defeated from Earth with help from both sides.
- Digimon Frontier flirted with the idea an attack on the real world in the penultimate episode's cliffhanger, but the Big Bad never actually made it there.
- In the 2003 anime version of Fullmetal Alchemist, alchemist technology making it into our world is the plot of The Movie.
- It turns out that Those Who Hunt Elves need to get back to their home dimension to prevent it from fatally merging with the one they've found themselves stuck in. So their quest to get sent back becomes all the more important.
- In MÄR, Ginta's world and MÄR Heaven are connected largely by Snow's existence as a clone created from a fragment of Koyuki's soul. The Orb is also the manifestation of all the evil in Ginta's world as well. His plan was to use the connection between Snow and Koyuki to rip open a giant portal between the worlds to conquer Earth as well. In the end, Ginta ends up saving both worlds and Snow and Koyuki merge into a single person. Note this is only in the anime, in the manga the connection is different and the Orb's plan... doesn't make a whole lot of sense.
- Planet Ladder features a girl who discovers that there are nine dimensional Earths engaged in war and are moving towards collision with each other. She is prophesied to be the "Girl of Anansi" who can save only one of these worlds, but journeys among all the worlds to search for a way to save them all.
- Sonic X: Towards the end of the second series, the two worlds are revealed to be merging into each other again, with the catastrophic effect of canceling out each others timelines and causing time to come to a stand still. The only solution is to send the Sonic characters home.
- Watashi No Messiah Sama starts off with Trapped in Another World, with occasional visits back home. Then monsters appear on Earth, and eventually Earth is threatened with destruction. Naturally, our heroes can't save just one of the worlds.
- Pretty Cure has this in every series.
- Almost subverted in the third issue of the JLA/Avengers crossover miniseries, both groups of heroes are unsure they want to restore their universes when they witness the Face Heel Turns, Heroic Sacrifices, Suspiciously Similar Substitutes, and With Great Power Comes Great Insanity that they will experience in the "correct" universe. Out of everyone Hal Jordan, who has the furthest to fall. Argues they don't have the right to play God with the two universes, causing the heroes agree to restore reality.
- The Captain of the Virtual Console has this as its plot. Gancena must save the video game worlds and her own.
- In We Are All Pokémon Trainers, when given the choice, the J-Team decides to have Dialga and Palkia fork the AU inadvertently created by Cyrus into its own separate universe, despite the fact that it replaced their original universe, because they deemed it to have just as valid an existence.
- The second half of Last Action Hero has Danny returning to the real world and bringing fictional action movie hero Jack Slater with him. Unfortunately, Benedict is there as well, and he's got the ticket to travel between movies and the real world to bring through anything he wants.
- Sort of done in MirrorMask. By saving the dreamworld, Helena also saves, symbolically, at the very least, her mother's life, and stops "her world" from being shattered. It's not as Narmy as it sounds.
- Technically, the titular heroine from May Bird series has to save "only" the Ever After world, but since all humans (including herself) will inevitably go there after death, this affect everybody on Earth as well. And the Big Bad is actively interfering with Earth, too, and it's only a question of time when he wrecks the Earth directly.
- The Magician's Nephew, when Diggory lets Jadis loose in London and then in Narnia.
- In Aaron Allston's Doc Sidhe, the protagonist is transported from the "grim world" (ours) to the "fair world" (Faerie), where he discovers a plot to subvert the connection between the worlds, threatening both. Both worlds are again threatened in the sequel, Sidhe-Devil.
- In the Everworld books, it would be a bad thing if the gods crossed over into the world of our own, or if aggressive human men with guns crossed over into the world of Everworld. Both of these dangers threaten to happen over the course of the stories.
- In Keys to the Kingdom, Arthur has to deal with threats in both the House and on Earth, particularly since if the House falls, so does the rest of the Universe.
- Mark Anthony's The Last Rune series, which features characters spending time in, and crossing between, Earth and the otherworld of Eldh. These include multiple villains and world-threatening dangers.
- Coraline, where the titular heroine must beat the Other Mother to save her parents and stop Other Mother from kidnapping children.
- Stephen King's The Talisman sees Jack flipping between our world and the 'The Territories' in an attempt to save his Mother from dying, this also has the side affect of saving the Queen in The Territories as they share a magical link.
- The Dark Tower, although in this there are many worlds and Roland and his companions must save all of them (since they will all be destroyed if the Dark Tower falls). However, Roland's world and Earth are the main ones that the heroes focus on.
- Most of So You Want To Be A Wizard by Diane Duane is spent in an alternate universe where the Lone Power has already put out the sun. At the climax, the protagonists make it back into their own universe, but the Lone Power follows them through the worldgate and they have to stop it...
- In Doc Sidhe, the Big Bad's plan involves merging the Grim World and the Fair World, effectively destroying both worlds, and it is up to Doc and his crew and displaced Grim Worlders Harris and Gabbie to stop him: especially as Gabbie is central to the Big Bad's plan.
- The Affably Evil villain in Everlost is trying to kill everyone and destroy everything in the real world so it will all be "preserved" in Everlost.
- In MythQuest, there is a god named Gorgos who lives to change and destroy mythology. He escapes into an advanced computer system where he has the power to alter the outcome of the myths. Minor changes only seem to cause a ripple effect in global knowledge of the myth, but when he tricked a character into almost causing Ragnarok, a large earthquake was felt in the real world.
- The Famous Jett Jackson: Referenced in The Movie:
Silverstone: If Kragg gets this world, it's only a matter of time until he comes for yours.
- The cornerstone of Fringe and its mythology is an Alternate Universe, and the protagonists of the show (from the prime universe) are using Mad Science in an attempt to Save Both Worlds after the first attempt at crossing from one universe to another resulted in a parade of soft spots, cracks in the walls of reality, breakdowns of the laws of physics, swirly energy thingies, negative earth wedgies, dogs and cats living together, etc. that threaten to destroy both universes. This is juxtaposed by the characters from the Alternate Universe, who believe the only way to save their universe is to destroy the prime one first (and they may be right). Or not, as it is revealed that the two worlds are permanently intertwined; therefore, the only way for any character to save their world is to Save Both Worlds.
- Arcana Heart: An angel with a God complex is trying to break down the barrier between the real world and the Elemental world. She almost succeeds.
- Sonic the Hedgehog has this happen twice. Once in Sonic Rush Blaze is trapped on Sonic's world with the Sol Emeralds. If the Sol Emeralds aren't returned, both worlds will be destroyed. Again in Sonic Rush Adventure, except Sonic and Tails discover they are trapped in Blaze's world. Eggman and Eggman NEGA team up at the end of the game to steal the Jeweled Scepter, which keeps the two worlds separated.
- Outcast: Opening the portal between the two worlds creates a black hole which will swallow up the Earth... unless you enter the alternate universe to figure out how to stop it. It turns out the alternate universe needs to be saved as well.
- Another Century's Episode 3 features a "save both worlds" plot, as some Applied Phlebotinum that exists in both worlds creates a dimensional rift, pulling the two Earths together due to gravitational forces. Also connects to Eureka 7 pretty significantly, since it's an Intercontinuity Crossover.
- Tales of Symphonia soon becomes this type. The Big Bad's Evil Plan alternates which world is in trouble but both ultimately have to be saved for either to have lasting peace.
- The Legend of Zelda: A Link Between Worlds: Link starts out trying to save Hyrule, but soon finds that its counterpart, Lorule, is in even worse shape. In the end, it looks as though Hyrule will be saved at the expense of Lorule, which would make this a subversion... but Link and Zelda then wish on the Triforce for Lorule's Triforce to be restored, saving Lorule, and making this Double Subverted.
- The Twin Worlds of Stark (i.e. our familiar Earth) and the magical Arcadia in The Longest Journey have been one and the same twelve thousand years ago, and the biggest danger they face is an uncontrolled collapse back into each other. In this regard, April isn't saving Arcadia only to learn that she also has to save Stark—much rather, she is saving Stark, only to learn that in doing so, she also saves Arcadia.
- Similarly, Amy from ZanZarah: The Hidden Portal saves the world of Zanzarah from the dark forces, but in doing so, also restores magic to her own mundane world, which had lost it long ago.
- One episode of The Super Mario Bros. Super Show brings Mario and Luigi back to Brooklyn, but also Bowser and company as well. They have to save Brooklyn and the Mushroom Kingdom from Bowser.
- An episode of the Dungeons & Dragons cartoon had the kids finally get back to their own world, but Venger followed.
- In Barbie and the Secret Door, the heroines must save the magic of all realms of Zinnia.