Save Both Worlds

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. Pretty much the inverse of Up the Real Rabbit Hole.

Anime & Manga

 * Roughly half the series of Digimon fall under this.
 * Digimon Adventure has the characters trapped in the Digital World in the first episode, return home for the Vamdemon arc, then go back to finish off the Sorting Algorithm of Evil.
 * 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.
 * 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.
 * Digimon Xros Wars trapped Taiki in the Digital World in the first episode, returning home for a single episode halfway through, and the final episode/battle took place in the real world.
 * Fushigi Yuugi
 * Several of the .hack// series; although technically they can go home (save for a few, like Tsukasa in //SIGN and any Vagrant AI character), the action doesn't take place there, and there's hardly an installment without players desperately pulling all-nighters to save the world/s.
 * Parallel Trouble Adventure Dual
 * In the 2003 anime version of Fullmetal Alchemist, alchemist technology making it into our world is the plot of The Movie.
 * 'Dragon Drive' - A video game leads to an virtual world that acts as a stepping stone to an alternate world. With Dragons.
 * 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.

Films

 * 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,

Literature

 * 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 Eart, as well. Not to mention that

Video Games

 * 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 arn't returned, both worlds will be destroyed. Again in Sonic Rush Adventure, except
 * Marche in Final Fantasy Tactics Advance is sure that, when he destroy the world he claims to be a fantasy and restore the "real" world, he claims that the characters from Ivalice without "real world" counter parts will live on in a copy of the Ivalice after he destroys it. No wonder so many people consider him to be a villain.
 * 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.

Live Action TV

 * In Star Trek TOS an ion storm switches three crewman with their evil duplicates who happened to be in an identical spaceship, orbiting the exact same planet and were about to transport at the exact same moment. This is a common occurrence in alternate universes on TV and in movies. No matter what the people are doing they are usually in the same jobs in the same places at the same time. This alternate universe was used in both Star Trek DS:9 and Star Trek Enterprise, which had the genesis of the evil empire in the evil universe by changing what happened in Star Trek: First Contact.
 * This was spoofed in South Park where characters from the "evil universe" come through (easily identified by their goatees and bad split screen) except the "evil Cartman" was a good guy and not a jerk.
 * In Stargate SG-1 they found themselves the central point for dozens of versions of themselves coming through from a seemingly limitless number of alternate universes. The only real differences (as they all had the same people in the same jobs and mostly the same as each other) was a reunion with a duplicate of a character killed off in our universe.

Web Original

 * The heroes of The Dragon Wars Saga must save the magical world they're stuck in to save Earth.

Western Animation

 * One episode of Super Mario Bros Super Show brings Mario and Luigi back to Brooklyn, but also Bowser and company as well.
 * Similarly, an episode of the Dungeons & Dragons cartoon had the kids finally get back to their own world, but Venger followed.

Anime & Manga

 * If a Digimon series doesn't play the Trapped in Another World variant of this trope, then they play this variant instead. Digimon is very big on this trope in general.
 * Digimon Adventure 02 has the characters able to freely jump between the worlds.
 * Digimon Tamers has them take a trip into the Digital World halfway through, and they spent around a third of the series overall there. Jumping between the worlds isn't as easy as it was in Adventure 02, though.
 * Digimon Savers is similar to Tamers in that getting to the Digital World is easy, but getting back can be substantially harder.
 * Digimon Xros Wars the Young Hunters Leaping Through Time appears to be set on following this course as well, with the addition of actively exploring places in Tokyo to find the portals into DigiQuartz.
 * Maze Megaburst Space
 * Corrector Yui
 * Kyou Kara Maou:
 * Paprika has the protagonists trying to save both the real world and the Dream Land.
 * Manga example: 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.
 * Occurs somewhat in Sonic X where Sonic's world is rather casually revealed to have originally been part of earth, but one world split into two as the result of some vaugely described phenomenon. Towards the end of the second series, the two worlds are revealed to be merging into each other again, with the catastrophic effect of cancelling out each others timelines and causing time to come to a stand still... The only solution is to send the Sonic characters home. Yeah, okay.

Films

 * Sort of done in Mirror Mask. 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.
 * In Thor, the eponymous hero saves Asgard (his world) and Juntenheim (the Frost Giants' world) from danger in the climax. One could even argue that he saved Earth as well since Loki was making plans to visit that realm when he was finished subjugating Asgard and blowing up Juntenheim.

Literature

 * The Magician's Nephew, when Diggory lets Jadis loose in London and then in Narnia.
 * The Neverending Story
 * His Dark Materials, not just worlds that Will and Lyra come from, but Philip Pullman's entire multiverse.
 * The Apprentice Adept series; Stile is a native of science-based Photon, but is brought to magical Phaze as part of a Gambit Roulette to correct an imbalance that would destroy both worlds.
 * Aaron Allston's Doc Sidhe and Sidhe-Devil both fight foes that could disturb both "the grim world" (ours) and "the fair world" (Faerie).
 * 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.
 * Keys to the Kingdom, 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.
 * Extremely weird example in China Mieville's novel The City and The City. Too good to be spoiled.
 * Coraline, where the titular heroine must beat the Other Mother.
 * 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.
 * 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...

Live Action TV

 * 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).

Video Games

 * Chrono Cross
 * The Kingdom Hearts series, with a plethora of worlds.
 * Played with, in this case. The universe/ multiverse of Kingdom Hearts is literally called The World, which our protagonists do have a place in. Every part of The World is equally threatened during their adventures, at about the same time that the world hopping begins.
 * A better example would be the storybook Winnie the Pooh, which magically houses a portal to Pooh-bear's actual dimension, the Hundred-Acre Woods. The Woods are a haven from the Heartless for both the player and for Sora. However, as of Kingdom Hearts II, it seems that outside interference has screwed with its occupants' memories.
 * The Longest Journey as well as its sequel Dreamfall.
 * The Legend of Zelda: Twilight Princess has this with the light and shadow worlds. Most of the time spent in Twilight is actually the normal world just made over to look like the other one, but...
 * Final Fantasy V, although it's really more
 * 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.
 * Metroid Prime 2: Light and Dark Aether.
 * Except Dark Aether doesn't get saved as much as it gets utterly destroyed, Samus.
 * Sudeki. Much like the Final Fantasy V example,
 * Tales of Symphonia soon becomes this type.  both ultimately have to be saved for either to have lasting peace.
 * Also shows up in Tales of Eternia, as an intended result of a Yin-Yang Bomb.
 * This seems the backbone of Marvel vs. Capcom 3 (it's even there in the subtitle!), in which all characters fight in sceneries from both Marvel-Earth and Capcom-Earth.

Web Original

 * A key trope in the web fiction serial Dimension Heroes, in which the Dimensional Guardians must find a way to stop two dimensions from colliding with one another.

Western Animation

 * WITCH
 * Code Lyoko
 * The Grand Finale of Danny Phantom.
 * And the episode "Livin' Large". "Reign Storm" might count as well.