A Sister Trope to the For Want of a Nail episode. While For Want of a Nail explores another fork in the road taken by a character, an Elseworld takes a well-known character and plonks them into a potentially wildly different location and situation. This can add some freshness to a character which allows them to act a different way than normal canon might allow but may also become an excuse to write professional Transplanted Character Fic of the Recycled In SPACE variety.
Daring writers trusted by loyal fans may do this kind of episode without any warning or explanation. Well regarded elseworld stories generally involve 1) either keeping the characters and their motivations recognizable despite the new setting and situations or 2) working within the confines of the new setting in order to get back to the original premise in a reasonable way.
Comes from the term used by DC Comics for these kinds of stories; they publish one-shots and Mini Series like this. Compare to Alternate Continuity. If a show is all Elseworlds all the time, you've got a Commedia dell'Arte Troupe.
DC's Elseworlds are sometimes grouped into six categories. These categories can be applied outside of DC Comics, of course.
Historical: The characters are transplanted into a historical context. Example: Transformers: Hearts of Steel (19th-century robots).
Alternate Real-World History: Some element of real-world history is different. Example: Batman: Holy Terror (where the US is a theocracy).
Alternate Fictional History: Some elements of the work's fictional history are different. Example: Friends, "The One That Could Have Been" (where Monica is fat, Ross is still married, Phoebe works on Wall Street, etc.).
Genre Graft: The work changes genre. Example: The Prisoner, "Living in Harmony" (a Western).
Fiction Graft: The work is melded with a famous work of fiction. Example: Superman: War of the Worlds.
Potential Future: The story is set in a potential future of the setting. This tends not to be this trope as we use it here (since it's not an alternate universe, just the future of the one we have). Often a Bad Future. Example: Heroes, "Five Years Gone".
In fanfiction this is known as an Alternate Universe (or AU), where the characters generally remain the same but the setting changes. High school AUs are very popular, probably because many of the writers are themselves in high school. (On This Very Wiki, we use a broader definition of Alternate Universe, of which Elseworld is a subset.) Not to be confused with Elsword.
Sora Wo Kakeru Shoujo's ninth episode plucks up the entirety of its main cast and sets them in a modern day world that tells a baseball story instead of the colony warfare one that had been playing out up until then. QT powers are still present despite this.
There are other Elseworld stories depicting the strawhats as mobsters, middle-aged women, and one where Chopper is a superhero, as well as a movie remake of the Drum Island arc with Robin and Franky already in the crew, along with the Thousand Sunny.
I, Joker is a one-shot about a dystopian future version of Gotham where people worship the current Batman (who is also called "The Bruce", but is NOT Bruce Wayne) as a god. It's told from the point of view of a person who believes himself to beThe Joker. This Batman likes to take enemies of the state, mind-wipe them, and turn them into carbon-copies of past Batman villains with implanted memories; he then uses them in a yearly bloodsport where the entire city dresses up as Batmen/girls/women and attempts to kill one of the villains so as to get a chance to fight him for the right to become the new Batman. However, after an act of rebellion from his personal doctor/surgeon who converts the rebels into faux villains, this year's Joker gradually regains his memories and, after discovering the original Batcave, defeats the wannabe Bat-god and takes up the mantle of the Bat. He also rescues his girlfriend, who had had her vocal cords removed as punishment for being a rebel; she becomes his Robin.
The first Elseworlds story is the criminally under-appreciated Gotham By Gaslight, in which Bruce Wayne is a young American plutocrat on a world tour in 1889, and ends up fighting (and is suspected of being) Jack the Ripper.
Speeding Bullets has Kal-El fall to Earth near Gotham City, to be discovered and raised as their own by the Wayne family. Or, "What if Superman was Batman?"
JLA: Act of God was a notorious one that involved all the people with inherent superpowers losing them.
In fact, it's implied (sometimes plain told) that every Marvel "What If" is one universe from the full Marvel multiverse. So, Marvel Zombies started as an alternate universe of Ultimate Marvel, then crossed with official 616-Earth. Ultimate X-Men have been spotted in Exiles, on a single panel showing scenes of the multiverse.
In its late 80's/early 90's incarnation, the series started relying on Outer Limits Twist endings. While What If? by definition tended to end unhappily, the second series relied on Villain Sue, Idiot Ball and ultimately stopped explaining the divergences' origin. One issue had Mephisto corrupting young Danny Ketch into a brutal serial killer without explaining why he didn't do this in the main universe.
Marvel also had The 5 Ronin, which transplanted Wolverine, The Punisher, The Hulk, Psylocke and Deadpool into Tokugawa era Japan.
The second-most recent generation of What If...? stories didn't really answer (or, for that matter, pose) a "what if" question, and were just, well, Elseworlds — like "The Devil who Dared", which features Daredevil as a ninja in feudal Japan.
Chuck Dixon's A Man Called Frank was noteworthy for being an Punisher elseworld when that was considered more of a DC trope.
Transformers: Hearts Of Steel was an Elseworld where the some of the Transformers wake up on Earth during the Industrial Revolution rather than in 1984 as they did in Transformers Generation 1, and took corresponding vehicle modes such as trains, propeller aircraft and warships. Human characters in the comic included John Henry, Mark Twain, and Jules Verne. According to writer Chuck Dixon this was meant to be a possible part of regular continuity, but numerous discrepancies (most notably the fact that the Transformers are seen waging war on Earth during the ice age, in the forms of fantastical creatures) contradict this.
Hearts of Steel was originally meant to be the first in a set of Elseworlds called The Transformers: Evolution. However, the series was never continued as Hasbro wanted to limit the number of alternate continuities (this was circa the 2007 film).
Durham Red: Scarlet Apocrypha took the eponymous character out of her far-future adventures and reimagined her as existing at various other places and times.
Back in the Silver Age, DC published "Impossible Tales" for Wonder Woman, in which she teams up with her Spinoff Baby selves (the Wonder Girl featured here is her teenage self, not Donna Troy who was introduced later) and Queen Hippolyta.
Keith Giffen wrote one The Authority spin-off story with Midnighter and Apollo as samurai.
The Doctor Who Magazine comic strip for Christmas 2010 was "The Professor, the Queen and the Bookshop", a version of The Lion, the Witch, and the Wardrobe (with elements of The Magician's Nephew) with Amelia and Rory as Lucy and Edmund/Polly and Digory, the Doctor as Professor Kirke/Aslan, the Rani as the White Witch (although her Sealed Evil in a Can form is a Weeping Angel), Azal the Daemon as Mr Tumnus, and the Talking Animals represented by Judoon, Cheetah People, Nimons, Hath and Silurians. At the end, it turns out to be a tale C. S. Lewis is spinning to the Inklings, the Doctor and Amy. The Doctor suggests it would work better with a wardrobe.
During "The Glorious Dead", the Eighth Doctor goes jumping through the lives of some of his Elseworlds counterparts, so we get him as a Wild West cowboy, a cartoon cat, a Doctor Strange expy, a Charlie Brown expy, etc.
The Beano Book 2010 had a strip called William the Cat, starring a Victorian version of their superhero Billy the Cat. It turns out to be All Just a Dream of the modern day Billy.
The Suske en Wiske comic "Het geheim van de gladiatoren" (the secret of the Gladiators) is entirely set in Roman times, with the 3 main protagonists as Gauls.
An episode of The Prisoner ("Living In Harmony") has Number Six up as a retired US Marshal in The Wild West, where a crooked judge tried to force him to become sheriff. (Of course, it eventually turned out that it was brainwashing.)
The "Benny Russell" episodes of Star Trek: Deep Space Nine in which Ben Sisko was thrust into a world where he was a 1950s science fiction writer (and possibly going mad), with the other characters recast as his friends and associates (or, for the baddies, racist authority figures).
In the Stargate Universe episode "Cloverdale", Scott hallucinated a world where the entire cast lived together in a small town, with Scott and Chloe about to get married. It kept the casts' personalities the same, along with many of the interpersonal relationships - for instance, Eli was Chloe's brother, while James was Scott's ex-girlfriend.
In the fourth season finale of Bones, Brennan and Booth are married, and all their friends and squints are either staff or patrons at their nightclub. This episode was probably the most polarizing ever to be seen on Bones, which is known for its consistency in tone, rivaled only by the last five minutes or so of "The Pain in The Heart". It was stuffed with clever in-jokes and references which would completely incomprehensible to even a casual fan, had a frankly awesome cameo by Mötley Crüe, and showed Brennan and Booth the way the vast majority of fans have wanted to see them from the beginning. But it was all a dream, and some fans were pissed because the sex between Booth and Brennan wasn't real.
An All Just a Dream episode of Smallville in which Jimmy Olsen imagined himself as the lead in a Film Noir. Interesting in that his subconcious apparently had a better idea of what was going on in the real world than he did; he later commented to Chloe how weird it was that a lot of his friends and associates (and the storyline of the episode) were accurately translated to the new setting, but then there was stuff like Clark secretly being a crimefighter, or Lana playing Lex and the good guys against each other.
In the Red Dwarf episode "Back to Reality", the crew wake up to find that they've spent the last four years of their lives in a Red Dwarf Total Immersion Video Game. It was all a shared hallucination.
Buffy the Vampire Slayer did one where Buffy is simply a schizophrenic young girl stuck in a mental institution, and not a super-powered monster-fighter at all, with her parents desperate for some means to help her regain her sense of reality. Interestingly enough, the ending of that episode left some doubt as to which world was actually the real one, and Joss Whedon himself has acknowledged that it's possible either could be.
On Las Vegas, "Everything Old Is You Again" is an episode in which the same characters operate the same casino, but in 1962. Even the opening credits were time-shifted, with shots of the characters in period outfits and the actual credits in a funky 60s font.
Bassie & Adriaan: this series frequently uses dream sequences to put the main characters in situations they can't encounter in the shows regular setting. They often start with Bassie wondering what it would be like if he and Adriaan would do a certain thing, and decides to have a daydream about it.
Supernatural has used this trope several times including the episodes "What Is And What Should Never Be" "It's A Terrible Life" "The End" as a Potential Future "The French Mistake" "My Heart Will Go On." In nearly all of these episodes the Winchester brothers must find a way to return to their reality and are or become aware they are not in their own world.
This is the premise of Penny Arcade Adventures, with the first storyline, On the Rain-Slick Precipice of Darkness, taking place in a Lovecraftian 1920's version of the Penny Arcade universe.
Borderlands 2 has the DLC Tiny Tina's Assault on Dragon Keep. Ostensibly, it centers around NPCs from the main story playing a knock-off of Dungeons & Dragons, but aside from the OOC-talk and goofy gamist mixups, it passes as a fantasy counterpart to the Borderlands world. With guns.
Infinite Crisis pits characters from multiple variations of the DC Comics universe against each other: there's the standard world, a magic world, a steampunk world, a nuclear holocaust world, a horror world, and a robot world.
In a way, all the arcs of Arthur, King of Time and Space are Elseworlds to the others. A bit different from most since there isn't one "main" universe (there's three).
MegaTokyo's various omake chapters are all this, sometimes combined with parody/homages as in "Full Megatokyo Panic." The whole comic, in fact, seems to be several Elseworlds mashed together, with different worlds visible to different characters.
The "Without Warning Or Explanation" type happened in Ben 10: The episode initially indicated somehow Ben went back in time to before he got the Omnitrix. He was then surprised to find that this time Gwen gets it, and he spends a good portion of the episode explaining to Gwen how the aliens work. The events of the first episode play out with these changes, and it wasn't an Elseworld episode until the very end when it didn't get resolved.
Futurama has the 2 Anthologies of Interest episodes, each with one of the 3 main characters using the Professor's "What-if Machine" which basically, when asked a question, shows a video of an elseword based on that question.
Benders questions where what if he were made 50-ft tall, and what if he was turned human. He ends up dead in both.
Fry's were what if he was never cryogenically frozen, and what if life was like a video game.
Leela's were what if she was more impulsive, and what if she found her real parents (played with in that she ended up knocked out and dreamed the elseworld, which is a Wizard of Oz parody.).
The first one starts with Farnsworth talking about his new invention "the fing-longer", which is essentially a cross between a glove and a pool cue. He demonstrates it by using it to activate the What-If machine, which everyone is (justifiably) more interested in. It ends with the revelation that the whole episode has been the Professor using the What-If machine to see what things would be like if he'd invented the fing-longer.
Various episodes of Pinky and the Brain would arbitrarily plunk the eponymous duo down in different historical eras, including the twenties, thirties, fifties, sixties, seventies, Napoleonic, medieval and biblical ages, among others. As the show tended towards Negative Continuity, no explanation was ever needed or given.
The "Darkwing Doubloon" episode of Darkwing Duck makes all the characters into Pirates. Except the Muddlefoots, they're the royal family of England.
Darkwing and Negaduck as space alien cousins. Also a parody of sorts.
Phineas and Ferb had a few. "Tri-Stone Area" (which featured all characters in a prehistoric setting), "Doof Dynasty" (which featured them in ancient China), "Excaliferb!" (which featured the characters in a medival/fantasy setting, although this was actually a story read to Major Monogram by Carl) and "Phineas and Ferb and the Temple of Juatchadoon" (an Indiana Jones spoof set in the early 20th century)
American Dad! did it twice. First with a James Bond spoof entitled "Tearjerker", where Stan is still a CIA agent but is fighting to defeat Roger, rewritten as a vengeful film producer named Tearjerker, who is planning to murder millions with a film so depressing it makes people cry to death. The second time, "Hot Water", was a musical episode where Stan buys a hot tub that turns out to be alive and psychotic. The episode ended with the hot tub murdering Francine and Stan. According to Word of God, "Hot Water" was written as a series finale because the producers hadn't received word about the show's renewal. When the renewal did indeed happen, they released the episode as a season premire (Albeit a non-canon one).
There was also a Christmas Special where almost everyone (including Stan, Francine, Roger, etc.) is left on Earth during the Rapture, except for a few people. The Anti-Christ takes over the planet, turns it into an apocalyptic wasteland, captures Francine, and Stan teams up with Jesus to save her. At the end, Stan goes to his own personal Heaven: home with his wife and kids implying the events actually happened and the rest of the series takes place here, until the camera pans to show Klaus mounted on the wall.
This episode got a prequel, where Hayley adopts a baby who turns out to be the Anti-Christ from the Christmas special, and Jeff dies while trying to help Stan and Roger kill it.
They've done a sequel episode to "Tearjerker" now, "For Black Eyes Only", where Stan and Tearjerker have to team up against a new villain played by Steve's principal (his name is simply "Black Villain").
There's also the episode "Blood Crieth Unto Heaven" which is staged as a play where the usual cast are all playing themselves or something acting out a "lost script" by some genius writer who was obsessed with the show. It's framed by a live-action Patrick Stewart watching from a theater box (despite the character he does the voice for appearing in the play itself).
The Powerpuff Girls did a few in the last season. There's one set in the wild west where they're the Steamy Puff Girls and one framed as a dream the Professor has where they were created without Chemical X and therefore don't have powers.