Sarah Connor: Look... I am not stupid, you know. They cannot make things like that yet.
Kyle Reese: Not yet. Not for about 40 years.
When humanity makes an enemy of a non-human race, we really know how to pick 'em. Not only are they unspeakably powerful, they're also really ugly! Eww! The possibilities are very varied. Maybe the aliens are horrid, reptilian creatures, noisy clanking machines or twenty meter tall giants. Of course, being so ugly and inhuman it's no surprise they've come to wipe us out. Well, the upside for humanity being involved in a (probably losing) war against these creatures is we can at least tell them apart from us humans easily, so they'll never infiltrate our society with spies and saboteurs. That is, until they figure out how to make a Mobile-Suit Human, make Ridiculously Human Robots, or use People Jars to change into human forms.
Which is what they just figured out: they look like us now.
The revelation will put the entire cast on edge, because they have just discovered the aliens/robots/monsters have developed a (near) perfect Masquerade or disguise. They aren't Puppeteer Parasite, able to take control of specific people or pose as them, just able to pose as a human. They are nonetheless much more dangerous because with enough patience they can now infiltrate any group.
Expect small groups to tear each other apart over mutual accusations of alien-ness... just what The Mole wants. For a measure of irony, the aliens infiltrating humanity to rip it apart from the inside may discover that Humanity Is Infectious... and not in a The War of the Worlds sense.
Most of the Shapeshifter tropes and spy tropes apply, with the caveat that things like Kill and Replace and Dead Person Impersonation can only be used if few people really knew the original, since the impersonator can't mimic specific people. A trope increasingly beloved by Live Action Series, as it neatly avoids the potential Special Effect Failure of making robots, aliens etc... and saves money on Special Effects.
Contrast A Form You Are Comfortable With, which doesn't involve the aliens deceiving us into mistaking them for humans; Human Disguise, for examples of non-human creatures passing themselves off as humans for purposes other than invasion; and Human Aliens in which the aliens just happened to look like us to begin with. Compare The Virus. Sister Trope to Puppeteer Parasite. A rich source of Paranoia Fuel.
- Inverted in Martian Successor Nadesico, when the Jovians — frequently referred to as "lizards" for much of the first half of the series and depicted as monstrous nonhumans in government propaganda — turn out to actually be humans, forgotten colonists from Earth.
- Robotech and its parent Super Dimension Fortress Macross have the Zentraedi, a race of giants who later reveal they can "micronize" to become our size. They send a team of spies into the SDF-1, which backfires horribly for their Planet of Hats culture.
- Genesis Climber MOSPEADA, another series adapted into Robotech, has the Inbit/Invid start recreating themselves in humanized forms, starting with one sent to spy on the protagonists.
- In Sgt. Frog, higher-ranked Keronian platoons are authorized to use technology that lets them mimic humans almost perfectly. Unfortunately for Keroro, his Rank F squad is forced to rely on a set of dubious robotic suits.
- The formless JAM in Sentou Yousei Yukikaze. In the anime, they start by copying Earth's technology (namely weapons and aircraft) before later moving onto human beings themselves. As it turns out, this is how the JAM see Rei and Yukikaze.
- This has always been the approach of the Skrulls, the antagonistic alien race found in Marvel Comics, but most of all in the Secret Invasion Crisis Crossover.
- Also, the now-gone Dire Wraiths from Rom Spaceknight.
- In the 1970s, the Avengers storyline The Kree-Skrull War featured a major panic over this, which (among other things) led to the formation of the Special Committee on Alien Activities. Ironically, it was not the shapeshifting Skrulls who were the main target, but their enemies the Kree—Who could still infiltrate Earth in human guise, because they were Human Aliens.
- This was eventually used as part of the Fantastic Racism angle in Marvel, to better justify the Supernatural Angst of the X-Men; mutants got a bad enough rep when they were all believed to be deformed and unsightly, but then the humans figured out that some mutants looked perfectly human whilst having superhuman powers...
- The Evronians from Paperinik New Adventures have shapeshifting Super Soldiers just for this before arriving on Earth. They're used only three times, but each time they're devastating:
- The special "The End of the World" shows that one of them, Kravenn the Hunter infiltrated the Xerbian government and was decisive in the vote that made the Xerbians temporarily deactivate their orbital defences as a sign of peace (the Evronians invaded in that precise moment and overran the planet in a few scant days). The same soldier, who also has a tracking superpower, continues infiltrating Xerbian pockets of resistance in the (failed) attempt to capture a member of the government, going as far as jumping on one of the three evacuation ships;
- late in the series, after Xadhoom (a Xerbian who wasn't on the planet for the invasion due an experiment that gave her godlike powers) has reduced the Evronians to a few scattered remnants, Grrodon (a shape-shifter of a different variant, with no additional abilities aside shape-shifting but considerably more intelligent and autonomous than the average Evronian) infiltrates the US Army to prepare an invasion. He's arrested as a spy... But he's never outed as an Evronian. In fact his first appearance was in an episode set in the twenty-third century, and he's only found out because Paperinik recognizes the signs of an Evronian weapon and he drops the disguise;
- The 2014 continuation "Might and Power" shows how devastating Grrodon could have been: he broke out of jail and built an Evronian army from scratch under everyone's nose. Had an intervention from the future not forewarned Paperinik, he would have conquered Earth in a month from the start of the attack.
- in the reboot, a two-parter starts with a group of shape-shifting Evronians infiltrating a military base. Late in the two-parter the Evronians are invading Earth en-masse and we suddenly find out that every single military force on Earth has been incapacitated, and thus Pk is the only resistance.
- In Clean Room both Astrid and Chloe are able to see through the demons' stealth abilities. The demons respond by possessing human hosts without controlling them, which makes them indistinguishable from unaffected humans until they attack.
- The use of humanisation technology in Pokéumans allows transformed Pokeumans to resume their old human forms - but only for a few hours. The PRT use these to rescue transforming Pokeumans before Pokextinction get there.
- In Mass Effect Human Revolution the Geth Infiltrator disguises itself as a human in order to get into Noveria.
- In Vathara's Walk Through the Valley has a rare heroic version. Hiko's people, the Confederacy, are at war with The Federation, which insists on tearing its women and children screaming from their homes, battering them and turning them over to the merrows for a form of Mind Rape called conditioning, which leaves you happy to do and be whatever the Federation wants you to be. It's implied that it leaves you without compassion. The reason they take slaves is to restore the genetic diversity they lost when they became chronically inbred because they were such a small, isolated population. They tell themselves that these gene donors are willing. They also do it to Confederate soldiers, who are dropped back into the units they were pulled from to wreak havoc on their comrades. The only way to prevent yourself from becoming conditioned is to temporarily become a Little Bit Beastly with the help of LEGO Genetics, which entails government-approved DNA sequences and extremely complicated life-support equipment that no one uses for anything else. Using claws, etc., means modifying the brain. Ensuring that alterants know when to use them means adding instincts from the species used as a source of DNA. This makes alterants easy to spot, no matter how well the alterations are hidden. They cannot make alterants in mind only; volunteers go mad in weeks. This is where the ookamimoya alterants come in. The ookamimoya are a species of EM-sensitive, lupine predators that live very long lives, have few offspring, which they cherish, a pack with which they share a Psychic Link, extreme distrust of outsiders, incredibly durable bodies, Innate Night Vision, Alien Hair in the form of sensory tendrils with which they pick up emotions and which move on their own; pelt tendrils that don't; and hidden claws. Ookamimoya alterants will have sensory tendrils on the scalp the thickness of human hair, eyebrows and eyelashes with tendencies of both pelt and sensory tendrils, zero hair or not-hair elsewhere, Kaleidoscope Eyes with exotic colors that shift to golden/amber at night/low light conditions, the ability to sense ki, lack of aging, hidden claws and a strong killer instinct that drive them to kill the conditioned.
- In Men in Black, the alien villains do this, but so do many (relatively) friendly aliens, as it is the only way they can live safely on Earth.
- Ditto Halloweentown.
- The eponymous robots in Screamers started off as small, subterranean weapons; they were deployed on a desert planet torn by war between rival factions. The screamers were produced en masse in an underground, automated facility, which eventually began to develop more advanced models; Type Is were still small robots, but more streamlined. Human models were also developed, including small children, a male adult, and a female adult. Becker, a Type II (adult male), claims he can change his appearance by carving off the faces of humans.
- In Terminator Salvation, this is the first time they go from "noisy clanking machines" to "Ridiculously Human Robots" in a Terminator film. Skynet explicitly went through several Terminator design evolutions to do this, reaching the fleshy T-800 and Shapeshifter T-1000 and T-X.
- Transformers: Revenge of the Fallen got a good amount of drama out of having the Decepticon "Alice" who could pose as a human. As if paranoia about everyday vehicles and devices wasn't enough.
- Inverted in District 9, when the young alien remarks that Wikus, in the process of transforming into an alien, "looks the same" as him.
- Mimic was basically about this — the giant mantis/termite hybrids may not have a perfect disguise, but if you're in poor lighting and not paying attention, they look a lot like a tall man in a trenchcoat.
- Referenced in Back to the Future, though of course Marty McFly is perfectly human.
"It's already mutated into human form!"
- The entire premise of The Thing (1982) and The Thing (2011), played for all the Paranoia Fuel it's worth.
- Agents in The Matrix take on human forms but are easy to spot due to their choice of attire. Also, in The Matrix Reloaded, Smith downloads their programming into human mind in the real world making them even closer to being human.
- The alien invaders in They Live use a cloaking satellite to disguise themselves as humans.
- Time Runner: The aliens infiltrate human society for several decades before launching an all-out invasion. Their number even includes the World President.
- They Look Like People is about a man who is either having paranoid delusions or is one of the few people to know about "monsters" who pass themselves as humans.
- Philip K. Dick did several straight versions of the premise (Screamers, mentioned above, is based on one of his stories), as well as a parody: in "The War with the Fnools", the Fnools are capable of an almost-perfect human disguise, but are still pretty easy to spot, because (a) they usually travel about in groups, all wearing the same disguise, and (b) they're only two feet tall.
- Terry Pratchett has the Auditors do this in Thief of Time: one takes the form of a woman much of the time in order to ensure the building of a glass clock that will stop time (and thereby make their job much tidier). When she starts acting too human, teams of other Auditors join her to ensure that it gets completed and started — while keeping her, and each other, under control.
- Clifford D. Simak has a group of aliens buying the Earth in his novel They Walked Like Men. The aliens were shape-shifters, capable of becoming anything at all — a human, an automobile, a pile of paper currency... Their default shape? Bowling balls. (Well, they looked like bowling balls, anyway.) To shift into something larger than their natural size, multiple aliens merged and changed shape.
- Calling All Creeps is pretty much based around this concept.
- Pops up several times in Animorphs, where the controllers suspect a human of being an Andalite in morph.
- The Mokuls who have infiltrated Earth in K. H. Scheer's Der Verbannte von Asyth (trans. The Exile from Asyth) explicitly use masks to disguise themselves as human and study prospective targets for quite a while before replacing them. (They also tend to keep the kidnapped originals alive in order to pump them for more information.) The eponymous protagonist and his companion, on the other hand, belong to a species of Human Aliens to begin with — though as it turns out their fingerprints aren't anywhere near human, and they have a few other peculiarities (like the effect coffee has on their metabolism) that end up giving them away.
- Witches in Roald Dahl's The Witches conceal their physical peculiarities to look like ordinary women, but there are still a few telltale signs including oddly colored eyes, oversized nostrils, blue saliva, and gloves to cover their claws.
- The Trope Namer is Battlestar Galactica (2003). The Cylons have evolved from "walking chrome toasters" into Artificial Humans. The opening and a few characters use the line, and it does cause a lot of mistrust among the human survivors.
- This is used from time to time on Star Trek, usually as a result of Magic Plastic Surgery, but other examples include the appropriately named shape-shifters from Deep Space Nine, and Species 8472 in its final appearance on Voyager. Inverted on an episode of Star Trek: The Next Generation where the Romulan proconsul correctly surmises that The Federation is using Romulan-looking spies.
- Also used in an episode of The Original Series, combined with Humanity Is Infectious.
- This trope is outright exploited by the Founders in Deep Space Nine: One reveals himself to Sisko to gleefuly reveal that there are only four Founder infiltrators on the entire planet Earth, center of The Federation, and look what they've accomplished! In fear of the possibility of Founder infiltrators amongst them, Earth declares martial law, and a series of events nearly leads to an outright military coup of the civilian government, and two Starfleet ships firing on each other.
- The human-form Replicators in Stargate SG-1. A different group of bad guys try to invoke this with a friendly species that create illusions and fake memories in the minds of potential hostiles as a self defense mechanism.
- V (both 2009 and the original) has reptilian aliens.
- Played both ways, as not only have the Vs infiltrated human society, there is a rebel faction which sympathizes with humankind and seeks to thwart their leader's plot. It's revealed that these rebels have even infiltrated the upper echelons of the V command structure.
- This is by far the most significant difference between the The War of the Worlds film/radio show/whatever and the later TV show — in the latter, the Martians are able to posess human bodies at will.
- The second season implements the same trope, but in a radically different way: the second wave of aliens have altered their biology to the point that they now look outwardly human (Albeit with luminous blood)
- The Outer Limits (1995): The nine foot tall, eight hundred pound Reptilian monsters with whom Humanity fights a losing war in a couple of episodes manage to pull this off by surgically-altering their (much smaller) females.
- The Skins on Roswell.
- Every alien except Cole and Zin on Tracker
- All the aliens in The Invaders.
- Parodied (with particular nods to Battlestar Galactica) in a sketch on That Mitchell and Webb Look. In the midst of a Robot War, a rumour spreads that robots have infiltrated the space station, and are supposedly indistinguishable from humans. Cut to a particularly clunky Tin-Can Robot, talking to several other people (in textbook Robo Speak) who seem to be under the impression he's human. Even when a slice of toast pops out of him.
- This is the stock in trade of the Zygons on Doctor Who.
- A comedy sketch on That Mitchell and Webb Look depicts a Replidroid (robots out to destroy all human life) named "Colin". They are considered to be indistinguishable from humans. Despite Colin saying "Does not compute", "Must eat electric" and "Must destroy humans, putting oil on his food rather than gravy, and being obviously made of metal, the crew are completely oblivious.
- Call of Cthulhu.
- The Mi-Go can surgically modify themselves to fit inside a "human suit" and masquerade as people. Said 'suits' are made by hollowing out a human body, by the way.
- Serpent People can use spells (such as Body Warping of Gorgoroth) to take human shape.
- In Traveller there is an interesting twist. When the Terrans meet the Vilani they learn that they actually are human. Of course that does not necessarily make the meeting pleasant.
- This trope is why, according to Task Force: Valkyrie from Hunter: The Vigil, werewolves and other shifters are a much greater national security threat than other supernatural creatures. Vampires have known limitations, and mages have difficulty using spells in public, but the Uratha look completely normal until they walk into a secure area and turn into eight-foot-tall killing machines.
- StarCraft II: Heart of the Swarm adds a (multiplayer-only) Zerg unit called the Changeling, which upon entering sight range of an enemy unit automatically changes its color and transforms into that enemy's tier 1 unit (Terran marine, Protoss zealot, or zergling). Since it can't attack and has no other abilities, this is only useful for scouting.
- A recurring theme in Super Robot Wars Original Generation. Even when not deliberately trying to disguise themselves, most of the alien monsters they fight seem to get progressively more humanoid as the game goes on. Actually inverted with the two main alien factions, though, similar to the Nadesico example. One turned out to be human abductees brainwashed by an alien Master Computer, the others (& by extension the creators of the aforementioned computer) were actually descendants of a lost starfaring civilization from Earth. Played straight by the Eldritch Abominations known as the Einst, however, who can create copies of people they've encountered & whose supposed Ultimate Lifeform is an enhanced clone of one of the main characters.
- Midway through Marathon, the Pfhor begin fielding Action Bomb simulacra that blend in with the Green Shirts... Except for their (mostly) strange exclamations and Alien Blood.
- In City of Villains you can contact Kelly Uqua, who is working for Crey Industries. There's a nasty rumor floating around that the real Uqua is dead, and a Rikti alien can alter its form to match hers. Kelly wants you to enter one of Crey's warehouses and delete this report. Kelly also occasionally slips into the Rikti's Verbal Tic while talking to you. A separate City of Heroes mission has you finding a Rikti who was posing as Kelly, whose transformation machinery has broken down.
- In XCOM: Enemy Unknown the Thin Man alien units would count as this. Their disguise is pulled off pretty good as well... thin men! Until they pull out their alien weaponry and start doing back-flips in business suits that is. Also their eyes if studied are reptilian in nature. Which would explain why they wear dark glasses. The bases of their necks are also scaled, implying that only the always visible parts are human in appearance, although this isn't stated ingame.
- ADVENT troopers from XCOM 2 look human enough until you remove their faceplate as happens during a cutscene and during any Alien Autopsy you perform on them. Then again, ADVENT troopers are peacekeeping forces and only need to look human enough to not be scary to the human populace while still being armored and armed well enough to keep order in the the Vichy Earth regime. Faceless, on the other hand, are straight up Shapeshifter that pose as civilians to ambush your troops while they are trying to rescue them (if they are aligned with La Résistance), or avoid them (if they are aligned with ADVENT, since the Gullible Lemmings they are will alert the authority if they see heavily armed terrorists walking around, but usually aren't attentive enough to pose a threat to the squad).
- The Thing (2002) has this as a gameplay mechanic. By using a syringe and a chemical that reacts to blood by releasing heat, you can test your party members for infection. If the syringe changes from red to brown, they're A-O-K, but if the syringe breaks, fry him, he's an alien, and mutating into a beastie as we speak.
- Xenoblade Chronicles X: For the most part, the city of New Los Angeles is a safe haven for the player where nothing bad happens during the early game. As the game gets more serious and darker, though, it's revealed that the Ganglion have an alien race known as the Definians on their side... who can perfectly and instantly shapeshift to look like anyone they please. Cue large amounts of paranoia among the characters after several Definian spies are revealed within the city... including one who has been there since before the start of the game.
- In Fallout 4, the Institute has reached a point in its work on autonomous androids that their current Generation Three Synths are functionally human, to the point the only way you can tell if someone is a Synth is to kill them.
- In Girl Genius, "revenants" were initially known to the characters by obvious mindlessness and a Zombie Gait. The readers have known almost since meeting the first one that some act perfectly normal until they receive an order, which came as a nasty shock when they were found out.
- In Stick in the Mud monsters usually take human forms when visiting human world, Rod's bar in particular.
- Partially Clips parodied this once.
- Instrumen from The Sanity Circus are capable of taking on the form of (empowered) humans, although Morphic Resonance applies in terms of clothing. The Scarecrows also appear as humans, with their black eyes and stitched mouths only appearing at certain (suitably dramatic) moments. Additional series art has suggested that these forms are nowhere close to being their true ones.
- In The Simpsons: Treehouse of Horror VII (during the short "Citizen Kang"), the evil aliens Kang and Kodos kidnap US President Bill Clinton and Senator Bob Dole, both of whom were the top candidates for the then-current 1996 presidential election. Kang and Kodos use a machine which disguises them as the two politicians, allowing them to run in the election and take over the USA (and soon the whole world). For some strange reason, nobody really notices how "Clinton" and "Dole" are behaving so weirdly and unearthly, and it's only until when Homer Simpson rips off their disguises in public does everyone see that they're actually aliens. Though this doesn't stop Kang and Kodos from telling the voters that they still had to pick one of them.
- Transformers in general is clearly in the same spirit. While most Transformers disguise themselves as vehicles rather than people, the principle is the same... Any car on the road or plane in the sky could be a robot in disguise. One of them could be in your driveway right now. And that's just the rank and file; it's been shown that they've branched out and there are Transformers who are capable of becoming all sorts of things you might have inside the house as well, like a camera, a microscope, a wristwatch, or a computer not terribly different from the one you're looking at right now. That monitor you're staring into? It could be staring back.
- Roughnecks: Starship Troopers Chronicles had the Imposter Bugs (a subspecies that could fit inside the Mobile Infantry's Powered Armor to evade detection), and later the Infiltrator Bugs (full-on shapeshifting in and out of human form).
- The Flintstones: "Ten Little Flintstones" has an alien ship landing in Bedrock and creating a series of Fred clones to learn about Earth prior to a pending invasion. The clones simply march around smiling vacantly and saying "Yabba-Dabba-Doo" ad infinitum, and everyone thinks each is actually Fred.
- Tom Terrific: The story arc "Million Manfred Mystery" dealt with some impressionable aliens who mistake Mighty Manfred the Wonder Dog to be Tom (he's wearing Tom's funnel cap while he's off at the store), so they make a plan sheet of Manfred and the entire alien race duplicate themselves as Manfred. The base reason is that Tom is so likeable that the aliens want to be just like him so Earth people will like them as well.