The cast has been infiltrated by a shapeshifter, replicant
, robot, or some other creature that is able to pass as human; or else someone formerly trustworthy has lost their humanity to The Virus
and is now secretly working against them. How do the other characters determine which of them is no longer human?
If they already know that the impostor has a certain type of Glamour Failure
or Kryptonite Factor
, then they can use that weakness as the basis of an Impostor Exposing Test.
If the impostor is an alien, they can cut themselves and see who has Alien Blood
. If it's a vampire, they can dip their hands in holy water and see who gets burned.
This can go down in a number of ways. Someone accused of being an impostor may simply perform the test on themself to prove that they're human. More dramatically, there may be a high-tension scene where all the suspects gather together and perform the test one by one. When the impostor is exposed by the test, or when its turn to take the test comes and it realizes that it has no way of avoiding being exposed, it will usually reveal itself and either attack the other people around it or try to escape.
However, the test isn't always foolproof: sometimes a very clever impostor will think of a way to either beat the test or make it look like a different person failed the test.
Compare Spot the Impostor
, where an impostor is identified using psychological means such as asking each person for a Trust Password
or Something Only They Would Say
Anime and Manga
- Near the end of Parasyte, the military figures out that people who've been taken over by the parasites can be identified by looking at x-rays of them — the parasites don't have skulls. They root out the parasites hiding among the people at City Hall by leading them past a large x-ray machine.
- Magic: The Gathering: In the novel Planeswalker, when Xantcha is accused of being a Phyrexian, she cuts herself to show that she bleeds blood rather than Phyrexian oil. However she actually is Phyrexian: as a sleeper agent, she was created specifically to be able to pass this kind of test.
- The Dresden Files: Murphy has Mort cut himself in Ghost Story before inviting him inside. A lot of supernatural beings that require an invitation to enter a building will bleed ectoplasmic goo rather than blood. The invitation, or lack thereof, is another such test in and of itself—an imposter using magic will either not be able to enter, or not maintain their disguise. Murphy pulls this one on Dresden himself, after, in a previous book, being attacked by someone taking his form.
- "Who Goes There??", the story that inspired The Thing (1982), used the same type of blood tests as the movie to identify the alien.
- In Sunshine, when Sunshine and Con are being interrogated by the police, Con is exposed to sunlight as they suspect of him being a vampire. He is a Friendly Neighborhood Vampire but Sunshine manages to use her magic to keep him from not frying and hence passing the test.
- In Jingo, werewolf Angua sneaks aboard 71-Hour Ahmed's ship in wolf form by posing as a Klatchistan wolfhound. Ahmed quickly catches her, however, by having the dogs eat from silver plates.
- Animorphs: The Unexpected. Cassie, hiding from the Yeerks on an airplane, tries to pose as a passenger. The Yeerks, knowing she's the only one on the plane who hasn't been affected by their paralysis-inducing phlebotinum, ferret her out by shooting everyone with low-intensity Dracon beams and seeing who flinches.
- In the Deryni novels, there's a drug called ''merasha'' that causes an immediate and violent reaction in Deryni but has no significant effect on "normal" people; during the persecutions, it was used as a way of uncovering secret Deryni. One application is specifically mentioned in the short story "The Priesting of Arilan": whenever a new priest was ordained, the communion wine at the Ordination Mass was spiked with the drug to make sure no Deryni got into the Church.
- Partially subverted in the German SF novel "Der Mann von Oros", where a blood test is used to dramatically reveal the shapeshifting alien whose frozen 'corpse' was taken aboard the vessel rescuing the surviving members of the stranded Pluto expedition...but fails to similarly detect a second alien who had already replaced one of the castaways on Pluto weeks before and thus had more time to perfect his disguise. This second alien is the 'man from Oros' alluded to in the title and the story's protagonist.
- Rather thoroughly deconstructed in Star Trek: Deep Space Nine. The shapeshifting changelings caused paranoia about their infiltration abilities. As such, Starfleet briefly initiated required blood tests of officers and their family, as any blood removed from a shapeshifter's body would instantly revert to protoplasm. Use of this test can be seen in the episode "The Adversary", among others. However, numerous problems with the test soon became apparent, such that a changeling could just kill some poor schlub and use their blood to pass the test. Ultimately, the person who proposed the testing was discovered to be a changeling infiltrator; he convinced Starfleet to use the test precisely because he knew that he and others of his kind could beat it. We even see one Klingon, who we later learned was a changeling the whole time, slit open his own hand.
- Also the Day of the Jackboot conspirators fake the test to frame Sisko as a shapeshifter once he finds out about their plan.
- Played straight the first time it was used ("The Adversary") in an obvious Shout-Out to The Thing; the shapeshifter escapes the moment it's asked to take the test; presumably a means of passing the test had not been worked out yet.
- In the Stargate SG-1 episode "Foothold", Melbourne cuts himself in front of Carter to prove he's human after an alien impostor is shown to have purple blood.
- When Sam comes Back from the Dead he ties Dean up so that he won't attack him, then cuts himself with a silver knife and swigs a mouthful of salty water to prove he's really himself.
- In another episode, a parasite has infected one of the characters, but they can't be sure who. They had earlier figured out that electricity was so effective on the parasite that it would be forced to leave the host, so the characters had to take turns shocking themselves to prove they didn't have it.
- In the seventh season, it's shown that leviathans have black blood. This trope is implemented when Frank and Dean have a small confrontation in which Dean cuts his arm to show Frank his blood. Frank does the same at Dean's insistence.
- In general, Supernatural has a lot of these, including drinking salt water (anti-ghost), touching holy water (anti-demon), cutting yourself with silver (anti-shifter), and touching borax or showing that you bleed red (anti-Leviathan.)
- Of course you need to be looking in the first place. When a demonic virus infects a town, the protagonists tie up a potential infected for several hours until his bloodwork clears him. After he's released, it turns out he was under demonic possession instead.
- Fringe: The sinister cyborg shapeshifters have mercury for blood. Blood screening is standard procedure when shapeshifters are at large.
- Star Trek: The Original Series:
- In "The Trouble With Tribbles", the Tribble dislike for Klingons is used to identify the Klingon spy disguised as a human.
- In an episode called "The Paradise Syndrome", Kirk plays along with aliens who think he's a god, until one of the aliens (who if memory serves were dressed like Native Americans of the Great Plains) cut him and contemptuously exclaims, "A god who bleeds!"
- In the Doctor Who episode "Smith and Jones", the Judoon have scanners which can distinguish humans from non-humans, which they try to use to find a plasmavore criminal hiding in a hospital. The plasmavore is able to change its physiology by drinking blood, and tries to use that to beat the test; however, the Doctor tricks it into feeding on him. Since he's a Time Lord, the scanners identify the plasmavore as non-human and kill it.
- Gaius Baltar spends most of the first season developing a Cylon detection test in the reimagined Battlestar Galactica.
- In First Wave, the Gua/human hybrid bodies the Gua use have built-in mechanisms that dissolve the body moments after death in order to hide evidence of alien presence. They also rapidly heal from wounds. When Cade wakes up after an explosion, he is told that the government now knows the truth and is starting a manhunt for the Gua. Cade begins to suspect something and, eventually, holds one of the agents hostage. In order to prove he's human, the agent sticks out his hand, and Cade stabs it. The wound doesn't heal. Later on, it's revealed it was a Gua operation, and the hand was deliberately engineered not to heal.
- Subverted in Sanctuary. Magnus, Will, and a few one-shot characters are trapped in a crashed plane in the Hindu Kush mountains with a shapeshifting abnormal. Realizing that the creature lives in incredibly cold climates, they decide to draw some blood from everyone and freeze it; the blood that doesn't freeze belongs to the imposter. They don't realize that the abnormal can make them see anything it wants, so the test is pointless.
- In the Red Dwarf episode "Psirens", a mind-reading shapeshifter takes on Lister's form. The crew manage to discover who is who by asking them to play the guitar. The Psiren reads Lister's mind, unwittingly picking up on Lister's delusion that he is a virtuoso, rather then a lousy player, and plays brilliantly. The Cat and Kryten then shoot him.
- Alias - "Project Helix" can make one person exactly like someone else, and there are three main ways to uncover the imposter. Of course, the imposters know all about what can identify them as imposters and spend lots of time on framing innocent people.
1) Provacillium, a medication taken by the imposters.
2) An eye test that analyzes proteins in the retina.
3) Out-of-Character Alert. "I just remembered: Francie doesn't like coffee ice-cream."
- The Voight-Kampff test in Blade Runner. It functions as an important tool for identifying replicants, as well as a unique and engaging game mechanic. The test automatically terminates after asking ten questions, regardless of whether a conclusive result has been obtained. The player can choose the nature of the questions, ranging from simple calibration questions (like those of a polygraph test), to enormously provocative questions involving child abuse and animal cruelty. In some cases, the questions the player chooses can be the difference between the test identifying the subject as a human or as a replicant.
- Spy checks in Team Fortress 2. Thanks to Friendly Fireproof, you can shoot your own teammates without hurting them, but if you shoot a Spy disguised as one of your teammates, they'll take damage. The Pyro's flamethrower is the classic choice, since only Spies will catch on fire. However, the spy has weapons in his arsenal which can enable short immunity to fire or fake a death.
- The princess in Shining Wisdom has been replaced by a demon however the real princess has a tiara that renders her impervious to damage so the only way to figure out who the real one is, is to attack her. This King is rather reluctant to do so.
- In real life this is known as a Shibboleth, after a bit in the Book of Judges where the Gileadites killed fleeing Epraimite refugees by pulling them aside and asking them to pronounce "Shibboleth"note Nowadays, it's become another name for jargon used among an in-group.