This is a rule guiding the creation of stories and plots, and is intimately connected to the principle of Willing Suspension of Disbelief.
The rule is, quite simply:
If it's required by your plot, make one fantastic assumption in your story and only one - and do it in or before the first chapter (or first page or two, for shorter works). Do not add more as the story goes on. And once you have your one assumption, all further fantastic elements must derive from it, not any new assumptions. And just to be fair, it must be either obvious to the reader, or something that can be deduced from evidence present in your story.
(And yes, having the whole point of a story being the process by which the reader finds or figures out the divergence which changed the entire landscape is perfectly valid, although if badly handled it can become little more than a Tomato Surprise.)
The rule and its name come from a set of writer's guidelines written by the late George Scithers during his tenure as the first editor of Isaac Asimov's Science Fiction Magazine between 1977 and 1982.
The name refers to the example of James Thurber's classic short story "The Unicorn In The Garden". Put simply, Thurber's story is about an ordinary suburban couple who wake up one morning to find that there is a unicorn in their garden. The story works because the only fantastic element is the unicorn. If on the second page a flying saucer had landed in the garden next to the unicorn, it would not have been as strong or as good a story.
This cannot be emphasized strongly enough to the writer, beginning or experienced: One and only one "unicorn" should be in play in a story. If you have two or more, you have a case where you need to find a more general "fantastic assumption" that allows for all -- or you have several different stories demanding to be written and colliding inside your head.
What constitutes "one element"?
That's the tricky part. It's "one element and its derivations". But a big story often starts from one leaf first and visits the root last. It may take many different ways there, often looking at a few leaves before showing how they are connected. Then at some point there's one introduced element, then another, then they become one.
Even when taking this too literally may be not feasible, the general principle still stands: multiple elements should form one system, then the "assumption" is this higher-level system. To look this way, it must show some sense or symmetry as a whole on a given scope, and the first component should be both meaningful (if not necessarily most important) for the system and linked to the next few. And the connection itself, whatever transpires to show it, in turn should tell something about plot, characters and/or setting. This road is navigable, though it adds new hidden pitfalls - and the shorter all connections are, the less pitfalls they need to dodge.
- Elements known primarily as parts of one mythos: if one kind of the Alchemic Elementals is introduced, the readers will expect three others to be at least mentioned and may even be puzzled if the author misses an opportunity to do so.
- - These can be considered components of a single bigger element, and further changes to it counts as new elements.
- Elements from closely related mythos: you may have an unicorn on the first page, and a dragon on the second - but why are they here together? There must be a strong connection between them, otherwise this will feel like a weird fantasy zoo.
- The specific interaction is arbitrary, as long as it establishes the connection: a fight to the death, posturing for the sake of appearances in the stalemate of an ancient conflict, one of them trying to save another, or simply a discussion of bloodlines that have the two in heraldry (to one - or better, both - of which an already-introduced character is related, of course). Even then, it's advisable to not introduce them one after the other. Let your unicorn act for several pages and establish its character; show its interaction with the scenery, establish that it's looking for something, is a Friend to All Living Things, is skittish, and so on. Or, introduce the unicorn and the dragon simultaneously, by dropping the reader In Medias Res ("heard them two fighting, approached to investigate" variant was used in actual epos), if this fits the pacing of the story or the start of their interaction needs to stay hidden for a while (here's your mystery - "over what their feud has started?" or "who exactly is that secret double-heir?").
- Either way, the result is that they become parts of a unified whole, so the assumption widens from "unicorn!" and "dragon!" to "a subset of mythological beast is real and acting now", which is less attention-seizing on its own, but more convenient for attaching arbitrary plot points that can do it too. (A "Cloak and Dagger" story with meddling by totem spirits of noble houses? This may be interesting.)
- - One may need to have the connections reinforced until it looks almost like the first case.
- One of the common modern variations is "vampires and werewolves"; while they don't appear together in the classics, and often seem to be conflicting re-interpretation of the same early stories, they share "gothic", "masquerade" and "transformation" traits in style. Note that when vampires and werewolves appear together, they usually have a strong pre-existing relationship, be it conflict, servitude, or comraderie in face of the monster hunters. Often they are given a Meta Origin. The setting usually doesn't need any relation - if they have different tastes and habitats, "what's Hecuba to him, or he to Hecuba?" - but the Mechanics of Writing strongly suggests it.
- Elements that are not already established as linked or compatible in any way: if you've got a unicorn on the first page and land a flying saucer on the second, the reader will expect that the premise is either Monsters vs. Aliens or "unicorns were pets of Ancient Astronauts!". The problems here: the first option may be seen as inherently goofy, and for the second the author needs to prevent Bait and Switch - otherwise those readers who wanted saucers may have already closed this story, and those who want unicorns may be put off by this little twist.
- - If the integration is done right, the story won't fail, but a typical result is the solution bringing in new issues, as above.
Note how moving the elements of premise apart toward a less-related mythos obviously makes it fail easier and harder, and adds challenges for actually pulling it off well.
Meta Origin is a typical mechanism for "cheating around" this principle by transforming multiple elements into subsets of one. Sometimes it works, sometimes it doesn't.
What constitutes "one story"?
At least, one arc. In fanfics or other short forms, it's the whole story. In literature, usually one book, in animation, one episode. See also Monster of the Week for a specific variety of one-shot single assumption.
- The gag-a-day format seems to allow more elements, like the full kitchen sink... but unlike kitchen sinks usually it strongly separates these elements - there is only one per episode. And the Negative Continuity makes this compartamentalization absolute. That's why gag-a-day routinely gets away with things that a single story could never afford without being reduced to either a complete mess or Idiot Plot. Tom and Jerry may have the characters subjected to Time Travel and run from a dinosaur, be abducted by The Greys and meet a Fairy Godmother - but it would happen in three different episodes, so there's no problem.
- Dragonriders of Pern has introduced dragons... and then slower-than-light spaceships, but on the Myth Arc level, constantly paving the road with step-by-step foreshadowing, and in part via Meta Origin. So yes, "dragons... and spaceships" turn can be done, but the turning radius ends up closer to "interstellar" than to "around the wingtip".
It's a subset of a general rule that if too many elements are intoduced at once, the reader will be dazzled or disoriented as to which is where. Unrelated elements tend to form a pile in mythological / fantasy / sci-fi / conspiracy / all-of-the-above kitchen sink. Should the reader need to dig through this all? Will the reader bother to do it? While the gag-a-day format seems to allow more elements, it usually also strongly separates these, so there's only one per episode. This principle dovetails into Like Reality Unless Noted, because the author has to figure out and describe differences from the "base setting", which usually is supposed to be our world. Also, this dovetails into The Law of Conservation of Detail. The introduced elements need some common framework and a way to resolve inevitable issues when they run into each other. Multiple independent authors can make it worse. The settings where All Myths Are True or All Theories Are True usually end up with either a Meta Origin or no continuity to speak of.
- Dungeons and Dragons. Yes, not even the big kitchen sink of kitchen sinks can afford to ignore this principle altogether.
- Planescape was made as one robust cosmology built upon a few simple and flexible principles. Sure, there's infinite customization that allowed to incorporate everything else - but under this, it has a very few basic principles, which is why it worked well enough. This emerged from the need for the existing weakly-related "otherworldly" elements to make some sense and was refined into a separate AD&D2 setting.
- D&D3 sometimes seems to have been built via showing the new designers older editions without explaining why this or that was ditched in the next book. Add to this the mixed blessing of Running the Asylum without as good continuity editors as before. So it began to lose this skeleton and slide into a "yes, but no, but yes" swamp - and soon there was no articulate cosmology at all.
- By the time of D&D4, cosmology turned into a compound of unrelated elements that seem to work better separately than together and end up either incomprehensible or ludicrous for the fans of any earlier editions. Much like the rest of it.
- Forgotten Realms has the Weave as the unified power source of all magic.
- Since it was already established that the Weave is damaged, the Shadow Weave was introduced later as the surviving broken-off (and hijacked) fragment. This massively Retcons the relative importance of many matters, of course. And introduces confusion. But at least it makes sense.
- Then along came the Demonweave, an attempt to plug yet another copycat into the same outlet, based on a pun (Lolth is a part-spider, and spiders spin webs. Or... weave them. Get it?) and no preexisting in-universe justification. If the fan site is any indication, this one fell quite flat.
- Gunnerkrigg Court began with the protagonist as a kid collecting oddities, proceeded to investigate those oddities and... glimpse by glimpse it draws a solid looking whole built around one element: the Spirit World. Almost everything else uses it, comes from it or feeds it, so the "new elements" are but variations of the existing ones. The only elements that seem to not fit in are treated as shocking or entertaining enigma in-universe. Beyond that, it's pretty much "All Myths Are True" by design, but what of it? After we know why it's so?
- Also, from the moment it mentioned a single Fire Elemental and on, the readers don't stop looking for the other sorts. This probably was inevitable.
- Shampoo's Revenge, a Ranma ½ Fan Fiction by Jared "Skysaber" Ornstead. This story starts with the "fantastic assumption" that a Ranma character - in this case Shampoo - can learn from her mistakes and formulate a plan making use of her canon resources which actually succeeds. It's a great idea, well executed, and carries the story along nicely for several chapters, until suddenly the reader is handed a new "fantastic assumption": that Nabiki has been so focused on enriching herself with petty con games, blackmail and betting pools for the last few years, she has completely missed a few details about her own home and family - like her older sister Kasumi being an Olympic figure-skating champion; that their mother is only divorced from their father, not dead; and that she has step- and half-siblings she never knew about because she has been too distracted by her schemes. Now either of these concepts would make a good story by themselves, but when they both appear in the same story, they compete with each other and eventually derail the whole plot - literally, as Shampoo's Revenge has been a Dead Fic since 2007.
- Hybrid Theory by Blade and Epsilon makes use of the rule as part of building its Deconstruction Crossover. It becomes a plot point late in the story that there are far too many "unicorns" loose -- the characters realize that with all the Crossover elements in the world, there are too many impossibly conflicting backstories and histories in play for their world to have actually existed before more than a few months prior.
The single introduced element may bloat out of proportion. And sometimes its exclusivity simply doesn't make sense. That's because the underlying principle is that the whole setting should be healthy as a system. The single element is easier to integrate into "Like Reality Unless Noted", but easier runs it over, too.
- Early "single gimmick" sci-fi had neither writers nor readers well used to introduction of elements that weren't previously established. So it ran on this principle, and thus demonstrates how it can be justified - and how clumsily it can be misused. This led to ridiculous results, such as sudden out-of-character Expospeak.
- Anything "futuristic" built this way. Just imagine "science fiction" of 2000-something year written in 1900-something that would extrapolate photography and telephones into videotelephones... but added nothing else, not even TV, and still runs on fire-belching steam automobiles. That's how all "one gimmick" worlds look like: bizarre and ludicrous. As if while the people who made jetpacks worked, everyone else stood around looking at them and did nothing on their own.
- With the Night Mail by Rudyard Kipling quite decisively breaks this principle, along with the usual approaches to Exposition, surprisingly so for early Science Fiction. Which is the main reason why it ended up brilliant in its way, if not one of his most popular stories. It has little in common with how e.g. Jules Verne or H. G. Wells wrote... and a lot in common with how people write SF now - except he didn't use established conventions and tropes, he built them from scratch.
Sanderson’s Third Law of Magic
Another side of this principle, as phrased in a weaker and constructive form by Brandon Sanderson: "Expand what you already have before you add something new."
This also covers approach to Meta Origins and other transitions toward a wider perspective: it works better if done gradually and somewhat synchronized with the plot, rather than suddenly and retroactively.
- Book of Amber had the 'verse turned inside out several times, but only via expanding what already was there, starting with only the Trumps, Pattern and Amber vs. Shadows.