They Look Like Us Now

ZentradiSpies.jpg in seamlessly and without drawing attention."

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 War of the Worlds sense.

Most of the Shape Shifter 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; 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.

Anime and Manga

 * 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, 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 Keroro Gunsou, 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.

Film
""It's already mutated into human form!""
 * 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 Shape Shifter T-1000 and T-X.
 * Transformers: Revenge of the Fallen got a good amount of drama out of having a Decepticon who could pose as a human. As if paranoia about everyday vehicles and devices wasn't enough.
 * Transformers in general is clearly in the same spirit.
 * Inverted in District 9, when the young alien remarks that . Also a Crowning Moment of Heartwarming.
 * Mimic was basically about this -- the giant roaches 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.


 * The entire premise of The Thing, played for all the Paranoia Fuel it's worth.

Literature

 * 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, others join her to ensure that it gets completed and started.
 * 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.

Live Action TV

 * The Trope Namer is Battlestar Galactica Reimagined. 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.
 * Galactica 1980 beats them by twenty-odd years by having human-form cylon robots in the episode "The Night The Cylons Landed". However, the ones on the reimagined series are completely human looking, down to having a reproductive system.
 * 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 three Founder infiltrators on the entire planet Earth, center of The Federation, and look what they've accomplished!
 * 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 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 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 nine foot tall, eight hundred pound Reptilian monsters with whom Humanity fights a losing war in a couple of epsiodes of the revival of The Outer Limits manages to pull this off by surgically-altering their (much smaller) females.
 * The Skins on Roswell.
 * Every alien except Cole and Zin on Tracker

Tabletop RPG

 * Call of Cthulhu.
 * The Mi-Go can surgically modify themselves to fit inside a "human suit" and masquerade as people.
 * The 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.

Web Comics

 * Partially Clips parodied this once.
 * 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.

Western Animation

 * 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.

Video Games

 * Starcraft II will feature a Zerg unit called the Changeling, which possess the ability to take on the shape of a human marine or a Protoss zealot.
 * 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.