Plausible Deniability

One possible definition of what differs between Science Fiction and Fantasy is the possibility of events taking place in "our" world. A Masquerade is required.

Thus, when Science Fiction shows us The Future, we are meant to believe that it is our future; which means that their past is our present, and therefore has to look like it.

According to this school of thought, if you want to do Science Fiction in the present day, you must maintain Plausible Deniability: it must be possible for the audience to believe that the events depicted "really did" happen, and they just didn't hear about it.

So, nothing can happen in the present which would make the six o'clock news. If you want aliens to show up, they can't land on the White House lawn and ask to talk to the press corps; they have to do it in secret. If you want to have super advanced Applied Phlebotinum, it has to be a top government (or industrial) secret, and can't be something you'd pick up at the local department store. If you want to have an Extraordinarily Empowered Girl, she needs to be sworn to keep her identity a secret.

This usually leads to a Government Conspiracy which keeps the average joe from finding out what's "Really Going On". You may alternatively have some magical or technological reason that these events are Invisible to Normals.

If your show is set in The Future, you get a lot more leeway, but you have to be careful when you talk about history. This tends to go horribly, horribly wrong when a show runs for a very long time and is rerun years later.

Despite the emphasis on Science Fiction being more believable than fantasy, this tends to show up in Urban Fantasy as well, where the magical elements are usually kept secret.

This is far from a universal trope, and, indeed, the gymnastics writers have to go through to have a long-running series maintain Plausible Deniability have become increasingly transparent. More and more writers have abandoned this notion, explicitly setting their series in an Alternate Universe. All the same, the urge is sufficiently strong that you may see an isolated lapse into Plausible Deniability in a show which does not normally bother.

Note that Superhero shows have not traditionally bothered. Instead, civilians largely accept, albeit with firm evidence, that the strangest things definitely exist and go about their lives hoping that the authorities and the superhero community can keep the more dangerous stuff under control. Instead, Arbitrary Skepticism is often used; Muggles may have no problem accepting aliens fighting mutants with the help of people in Powered Armor, but anyone who claims to be a God is clearly a fraud, or simply crazy.

Enforces External Consistency. Not to be confused with Implausible Deniability. Compare with Weirdness Censor and The Masquerade. Contrast The Unmasqued World.

Anime and Manga

 * Subverted big time in Kashimashi: Girl Meets Girl: no one will let Hazumu keep anything a secret even though early on she tries. The aliens announce what they did to her, and when she tries to hide Jan-puu the next morning she goes downstairs to discover her friend having breakfast with her parents.
 * At first Gainax's anime Nadia, Secret of Blue Water seemed to use this trope. It took place in 1889 and features remnants of Atlantean civilization fighting a covert war right under the world's nose, without seeming to affect history in any way. However, apparently at the end of the series they just said "screw it" and
 * Maintaining Plausible Deniability is one of the main conflicts of the Haruhi Suzumiya series. Itsuki (and, to a lesser extent, Kyon) likes the world the way it is, and makes it one of his goals to keep Haruhi's Reality Warping from changing things too much.
 * The Transformers series of various stripes tend to use this a lot. Especially Robots In Disguise, where one character is pretty sure she's going insane simply because all these strange things she seems to wind up in the middle of can't possibly be happening.
 * Oddly enough, given that "Robots in Disguise" is a series catchphrase, the Transformers rarely stay hidden and have usually outed themselves to humans within the first couple of episodes/issues in each continuity.
 * Revenge of the Fallen

Comic Books

 * In John Ostrander's The Spectre, it was established that there was a sizable minority of people who thought that Batman and Superman were myths.
 * Batman tends to prefer that ordinary people think of him this way; the majority of regular folks in the DCU (outside of Gotham, anyway) tend to think he's a myth.
 * In the first Suicide Squad series, Shade the Changing Man told about an incredibly confusing conspiracy that was going on in his home dimension. When Shade and the Squad confronted the conspirators on Earth, one of the Squad members asked what to do when the police arrived. Shade replied to tell the police the truth and they will brush it off as a delusional fantasy.
 * In the first Ultimates series, 20% of the American public believed the Skrull alien invasion was a hoax created by SHIELD to secure more government funding.
 * Naturally, that number jumped dramatically when the public learned that SHIELD had suppressed public knowledge of it's connection with the Hulk Disaster.
 * In the first Ressurection Man series (taking place in the DC Universe), our titular character confronted a bunch of mobsters and crooked cops while appearing in a monster-like form. One of the witnesses was later confined to a mental institution for reporting what she saw.

Film
"President Whitmore: Why the hell wasn't I told about this place? Secretary Nimziki: Two Words, Mr. President. Plausible Deniability."
 * The movie Men in Black (and the later animated series) is all about the people who maintain the Masquerade, covering up evidence of aliens on Earth. It helps the agents that they have memory-erasing technology and such, so that they can be the first and last people to hear reports of alien activity.
 * Lampshaded in Independence Day. The Secretary of Defense of the US keep the existence of aliens at Area 51 a secret even from the president so that if anyone asks, the president can't be caught lying and accidentally reveal that he knew.


 * Though it went into Wallbanger territory when he still didn't reveal it when the aliens actually turned up, but waited until the President had actually denied there was any previous alien contact.
 * Not necessarily. Nimziki thought that conventional weapons were enough to defeat the aliens. He was probably hoping he'd never have to tell the President about Area 51.

Literature

 * Thanks to memory charms, non-magical people (Muggles), do not know about about wizards and witches in Harry Potter.
 * The same thing also occurs in Artemis Fowl thanks to mindwiping.
 * In the classic science fiction story "What's the Name of That Town?" by R. A. Lafferty, Chicago has been destroyed in an unspecified catastrophe. The event was so traumatic that the very existence of the city has been wiped from historical records and everyone's memory. A sentient computer figures out the truth from a collection of disconnected clues, but the moment it has finished telling the real story to its human companions, the facts instantly once again disappear from everybody's mind and the computer's database.
 * Averted by Stationery Voyagers without explanation for most of the worlds of Inktacto. Then again, Inktacto is part of the Grapharino Galaxy, and Grapharino's Physicalia is an allegorical universe not unlike Middle-earth's (or even Narnia. Only difference, really, is more Science Fiction-oriented plot elements and way fewer High Fantasy ones. So Alternate History helps cover some tracks.
 * Subverted in-universe on Neothode, as the population is so used to being bullied by Astrabolo's cult that they are too far beyond the Despair Event Horizon to care about whether the weirdness in their environment is covered up or not.
 * Inverted in-universe on Mantith: The Bindaf 3000's landing on Mantith was actually expected, with Strawmen Political from every ideological camp trying to exploit the situation to their cause. It was the Voyagers who were caught off guard! Even for those causes the Voyagers agreed with, their insistence on wanting to stay on task rubbed many the wrong way.
 * In The Hitchhiker's Guide to the Galaxy, there is a galaxy-wide system of government, and the only reason we don't know about it is that we aren't as advanced as everyone else yet. In fact, there are a few aliens living on Earth, and a handful of humans know about them, but they have no connection to any Earth government. And the Earth has been blown up on one occasion, but it (and everyone on it) has been replaced with the same memories up until shortly before the world ends, so nobody remembers it.
 * Averted in Charles Stross's multiple-parallel-universe "Merchant Princes" series. For the first few books, everything could plausibly be going on under our noses, with strange events being passed off as hoaxes or terrorist attacks. He blows the lid off the Masquerade at the end of book five . (Also, it's revealed in passing that the main universe isn't ours, but a slightly different one—in which, for example, Saddam Hussein is killed in a coup just before the U.S. invasion.)
 * In the Percy Jackson series a magical force called the Mist shifts events involving demigods into something more mundane that they can process.

Live Action TV
""Never mind.""
 * Star Trek has had some bizarre run-ins with Plausible Deniability as a result of the original series's occasional mentions of late-20th-century "history". The intercontinental war of the 1990s is suspiciously absent from Flash Back and Time Travel episodes of Star Trek: Voyager, and Star Trek: Deep Space Nine appeared to have retconned that war into the late 21st century, though Star Trek: Enterprise finally gave up and admitted that the 1990s of the Trek Verse differ massively from those of our universe. (One difference is that apparently there was no TV show called Star Trek in the Trek Verse, as evidenced by the Enterprise crew walking around San Francisco in "The Voyage Home" without being mobbed for autographs... of course, this means that the Enterprise was named after the Space Shuttle, which means, in turn, the Space Shuttle must've been named after nothing in particular any of eight ships in the US Navy.)
 * More likely any of the fifteen ships in the Royal Navy, given that two of the others were named after British exploration ships (note the spelling of Endeavour on the shuttle's side).
 * No. at least not the Enterprise, that was named after The american ships for sure. we dont have much of a tendency to name our cool stuff after other countries ships. there other two, possible but unlikly, seeing as discovery and endevour names are words about exploring. more like they were both named after the same ideas, not each other.
 * Reading the other wiki - the Endeavour was named after HMS Endeavour, Discovery after not one but four British ships named Discovery, and Challenger was named after HMS Challenger. So, half of the shuttle fleet was named directly after British ships. Can't find any source for where NCC-1701 got its name from, though.
 * The Spin-Off novel series The Rise and Fall of Khan Noonien Singh made a heroic attempt to explain how the Eugenic Wars could have taken place in the real 1990s without anyone noticing.
 * War of the Worlds takes the unusual stand that most of humanity simply doesn't remember the massive and very public invasion of the 1950s. There's no major government coverup, and most humans could probably find out about it if they really tried, but most people just find alien invasions too far outside their normal sensibilities to think about it very much.
 * Power Rangers rarely bothers with Plausible Deniability, but there are a few oddball examples of trying to shoehorn it in: in "Trakeena's Revenge", a receptionist tells a small girl that there's no such thing as monsters, even though they've been attacking the city weekly for months and other cities for several years. In "Prelude To A Storm", Tori thinks the Power Rangers are fictional, even though they're major cultural icons by this point. (The producer later explained that he just thought the line was funny and didn't mean for fans to take it so seriously.)
 * When they crossed over with the TeenageMutantNinjaTurtles, each team thought the other wasn't real.
 * The entire nine-year run of The X-Files depends on the creators' abilities to maintain this trope.
 * Stargate SG-1 does this a lot. It makes sense for the public to not know about the Stargate Program itself, which is a secret government project, but fleets of alien ships attacking Earth (which should be seen by astronomers, at least) and strange events up to the teleportation of a whole building into space somehow are never noticed. (With that last one, the media even mentions that no explosion was heard and no rubble was seen, but they can't figure out what did happen.)
 * Astronomers DO see the alien ships, but the government tells elaborate cover stories, and most people choose to believe the stories and think that they misinterpreted what they saw other than accept the reality that aliens do exist. The ones who don't turn conspiracy-theorist and attract the attention of the government, who in that case tell them the truth and have them sign a confidentiality agreement.
 * Actually the fact that someone saw the battle (and is blackmailing the goverment with its existence) is the plot of one episode.
 * And then there's the time an amateur astronomer spots an asteroid on a collision course with earth and is trying to convince a switchboard operator to transfer him to somebody in authority when a big car with tinted windows pulls up and men in suits and sunglasses come pouring out.


 * Human ships used in Stargate are so large that, when in Earth orbit, they would be clearly visible to the naked eye and resolvable with amateur-grade telescopes. No conspiracy would be able to cover up an object in the sky only outshone by the Moon and Sun.
 * Or a gigaton nuke detonating in space. Then again, the blinding flash is really the least of your worries when a nuke goes off that close to the magnetic field....
 * Though given the vast number of people that now know about the Stargate program (it's been leaked on TV by a - then discredited - media mogul; all the Permanent Members of the UN Security Council have been informed, including China who have explicitly said they have no intention of keeping the USA's secret; an entire US Carrier group was sent to deal with Anubis' attack on Earth; they were partly responsible for the forced resignation of the US Vice President Kinsey; not to mention all the random "ordinary" people who've been involved in one episode or other) it's frankly ridiculous that the story hasn't got out yet.
 * And the carrier and cruiser were destroyed (as was possibly the rest of the battlegroup), good luck on explaining that one away (over 8,000 personnel, and over 150,000 tons of equipment).
 * "Meteor Shower"
 * What about all the US military personnel stationed offworld? Why hasn't someone noticed that the number of troops shipped out to say Iraq or Afghanistan is not equal to the number of troops that actually exist? Not to mention how to explain away all the casualties caused by the Monster of the Week.
 * Speaking of astronomers, some of them really should have noticed that the outside universe "jumped" forward in time several months, because the earth, and several other stars, looped in time for that amount. We don't know how big that effect was, but it's been several years, so unless the "bubble" included neighboring stars, light from outside it should have already hit the earth.
 * It did include neighboring stars, so perhaps the light since then from the stars that weren't in the bubble hasn't reached Earth yet, in which case it wouldn't become an issue for decades, or possibly centuries.
 * When in doubt, turn to the one explanation that excuses all space-based activity: "Meteors did it!"
 * Which might explain the finally of Stargate Atlantis. A massive cityship crashlands into the pacific, going over the entire bay area.
 * In Sabrina the Teenage Witch, the Witches Council had a rule that witches (and other magical beings) were not allowed to let mortals know the existence of witches and magic. With a few exceptions, mortals who somehow found out either underwent Laser-Guided Amnesia - or were made to believe that what they experienced was All Just a Dream.
 * Buffy the Vampire Slayer. Gas leaks. Gangs on PCP.
 * " Mayhem Ensues: Monsters Definitely Not Involved"
 * "I must've fallen on a barbecue fork..."
 * Buffy at least has a good reason for such a strong cloak of secrecy the Mayor/s Wilkins I, II, and III (same guy) built and carefully developed the town to fuel his own ambitions, mainly becoming immortal and trading sufficient amounts of regular townspeople to various demons to allow himself to reach ascension. He undoubtedly controlled the police and made sure to make the town appear low key enough to keep federal authorities away.
 * The general public in Lost is unaware of the strange events that occurred after the crash of Oceanic 815 due to the Oceanic 6 creating a complex cover story. This lie is not perfect, however, and the Mysteries of the Universe and Oceanic 6: A Conspiracy of Lies specials suggest some conspiracy theorists have begun to suspect something's up. It's unknown what effect the  had on this masquerade.

Video Games

 * Mass Effect: After the protagonist is put into an organization that is Judge, Jury, and Executioner, Big Good Admiral Hackett will repeatedly contact the Commander about helping the Human Systems Alliance clean up things that aren't entirely legal. He often states that the Alliance would never do such a thing, before sending you to clean things up. These things include: Supporting a drug lord with arms (Mission: Trojan Horse Assassination), sending reconnaissance probes that will detonate with nuclear force if found (Mission: Recover/Destroy probe), and doing illegal AI research (Mission: Destroy).

Webcomics

 * Although the aliens in The Inexplicable Adventures of Bob do have a (very loosely enforced) "hands-off" policy regarding Earth because it's a "nature preserve," the main reason that Generictown has not been swamped by the press and other curiosity-seekers seems to be an overwhelming, near-universal Weirdness Censor among most of the people in town.

Western Animation

 * Completely, totally, and utterly subverted in Codename: Kids Next Door. At first, the series seems like it could, with some Willing Suspension of Disbelief, take place in our world: the title organization is limited to a few kids playing in their leader's yard, and there are generally no credible witnesses around the kids' adventures, which could merely be attributed to their overactive imaginations. Then in the first season finale, Applied Phlebotinum is used which forcibly turns a character into an adult, and the Big Bad is revealed to have control over fire somehow. Even then, since that episode takes place mostly in the aforementioned villain's mansion and the characters never speak of it again, one could suppose that there's just some kind of Masquerade going on. After that, however, it's revealed that there are Kids Next Door operatives all over the world, that Sector V is just one small part of the organization, and that the 2x4 technology actually works and can easily ignore the laws of physics, up to and including building functional supercomputers, and having a friggin' MOON BASE accessed and operated by children, among other things. The villains' schemes also become wider in scope and more public at this point. Yes there is the Masquerade element that the Kids Next Door erase thirteen-year-old operatives' memories with plungers, but since the villains have no reason to enforce it, and considering all the strange and impossible places the characters visit (at least one of which, an ocean of asparagus, is in no way hidden and right next to a residential area), it eventually becomes clear that the series Never Was This Universe.