Emergency Impersonation

A secondary character of some importance in the world at large has disappeared or been rendered incapable of performing his usual duties: they've been kidnapped, or have run away, or they might have even died. Someone else (usually one of the program's leads) finds themself dragooned into service; they must impersonate the missing person during some critical event or meeting...or for much longer.

While sometimes the impostor receives sufficient briefing and/or real-time aid to bring off the deception seamlessly, it's more common for time or other constraints to prevent this seemingly necessary step. In a comedic setting, naturally, Hilarity Ensues; in more dramatic locales, the results can be anything from painfully embarrassing to potentially fatal.

Eventually the missing person (if they aren't deceased) is recovered, but often only after they learn An Aesop about some aspect of the world which was hidden from them in their usual role.

Compare with Prince and Pauper and Decoy Leader. Closely related to El Cid Ploy and You Will Be Beethoven.

The former trope name comes from the title of an episode of Angel which used an inversion of this plot, wherein the title character went missing and was impersonated by a member of the supporting cast. It might also be considered a subversion, as the missing character ended up learning absolutely nothing.

However, the fact that this plot comes from the classic novel, The Prisoner of Zenda, makes it at least Older Than Radio.

Usually -- but not always -- involves an Identical Stranger. See also Body Double and Mock Millionaire.

Anime and Manga

 * In El Hazard the Magnificent World, Makoto Mizuhara is forced to impersonate the missing Princess Fatora until she is rescued. He pulls this off with surprising ease.
 * In episode 12 of Mai-Otome, Mashiro runs away rather than meet a foreign prince, and Arika is forced to impersonate her. However, in a double invocation of the trope, the prince is himself an impostor, the bodyguard of the true prince who himself is missing. Naturally, Mashiro and Prince Takumi meet, neither knowing who the other is. (Pleasantly enough, this is not played for laughs, despite the strong tendency of anime using this trope to do so. What is played for laughs is Nina claiming to be Arika just for the hell of it -- and to further perpetrate the joke, Mashiro introduces herself as Nina, completing the circle.)
 * Inverted in an episode of Sailor Moon, where Usagi is kidnapped and Minako must pretend to be Sailor Moon. Everyone else is fooled, but Usagi is less than impressed.
 * Subverted in the anime Pani Poni Dash. Form teacher Rebecca Miyamoto disappeared just as the PTA are about to sit in during her class and her loyal students all but press gang Serizawa Akane to be her replacement or as Akane herself thinks, a "kagemusha". The subversion is that
 * Utilized in the manga Princess Prince, where Half-Identical Twins trade places after one gets pregnant. The twist is that not only is the "replacement Princess" a boy, but his "fiancee" is well aware of this.
 * In Death Note, this is played with - all the relevant major characters know about the deception, and it's all in all a pretty twisted case of the trope.
 * Specifically, Light takes on the guise of.
 * This is the entire plot of the manga Magical X Miracle.
 * In Tentai Senshi Sunred, Sunred receives the outfit of Sakyun, a fellow toku hero, back from the laundromat. He decides to beat up Florsheim, the local villain group, for the hell of it. It's only when ink is sprayed on the suit that Sunred realizes he has to return it to its rightful owner. This is the closest Sunred ever gets to acting like an actual toku hero.
 * In the Detective Conan vs Lupin III Crossover TV special, Ran turns out to be a near ringer for the princess of a small European country.
 * In The World God Only Knows, Elsea has to fill in for the idol Kanon

Comic Books

 * Happens accidentally to Shakira in an issue of The Warlord. Shakira discovers she is a dead ringer for a kidnapped princess. She takes the princess's place, leading the kidnappers to believe the princess has escaped.
 * Jon Sable Freelance #49 is an Homage/Whole-Plot Reference to The Prisoner of Zenda with Jon standing in for a kidnapped European monarch.

Film

 * The plot of the movie Dave, where the title character takes the place of the President of the United States after the latter unexpectedly suffers a stroke while having sex with an intern. Dave has to fool not only the country but the late president's wife, for Rom Com reasons. At the end,  By which we mean
 * The cult favorite Bubba Hotep relies on the premise that in the early '70s, Elvis Presley traded places with an Elvis impersonator; the impersonator died in 1977, and the real Elvis lived to a ripe old age in a retirement home.
 * Premise of the 1988 movie Moon Over Parador. An actor who has just finished filming a movie on location in the fictional Latin American Banana Republic of Parador gets coerced into pretending to be that country's dictator (whom he resembles and was already good at impersonating) when the latter unexpectedly dies.
 * Mel Brooks' film History of the World Part I uses this trope near the end, when the urinal boy replaces Louis XVI (both played by Brooks). Origin of the phrase, "It's good to be the king." Or, at least, origin of its entrance into pop culture.
 * This is the premise of the Wayans Bros. movie White Chicks.
 * The premise of the movie Bad Company, in which a dead CIA agent posing as a nuclear arms dealer has to be impersonated by his Separated At Birth identical twin brother.
 * This is the basic premise for Akira Kurosawa's movie Kagemusha, where the thief disguises himself as the warlord.
 * In the Bollywood movie Don, the protagonist, Vijay, has to go undercover to impersonate the dead eponymous character, a leader of an underworld gang in Mumbai, in order to help the police arrest the rest of the gang. In The Remake,
 * Happens in The Great Race (as part of The Prisoner of Zenda parody) with Fate being a dead ringer for the king of Pottsdorf.
 * In The Passenger Jack Nicholson's character assumes the identity of an acquaintance for reasons that remain difficult to fathom, even keeping meetings with men who turn out to be quite dangerous and who expect him to provide things he cannot.
 * Most of the plot complications in A Very Long Engagement are caused when the identities of two young WW 1 soldiers are switched. The character involved, Manech, has lost his mind and isn't really able to clear things up - it's the people around him who, for their own reasons, practice the deception.
 * Happens in Hair (the movie): George Berger impersonates Claude Hooper Bukowski to extract Claude from the base for a last meeting with Sheila, taking his place, but while Claude is away,

Literature

 * This trope is central to the story of The Prisoner of Zenda, where Rudolph Rassendyl has to impersonate his royal cousin who has been kidnapped the day before his coronation.
 * The story is parodied in Royal Flash.
 * And reused in the Robert A. Heinlein novel Double Star, also known as The Prisoner of Zenda...In Space!
 * The Discworld novel Hogfather does this, with impersonating the Hogfather (the local equivalent of Santa Claus). It's also the plot of Mort, with Mort impersonating.
 * Conan the Barbarian: In the only Conan novel written by the character's original creator, The Hour of the Dragon, Conan is incapacitated on the eve of battle by a wizard's servant. In order to keep his army from being demoralized and consequently losing his kingdom, he has one of his most promising commanders don his armor and lead his men in his place--only for the commander to have a cliff face dropped on him, making the world at large believe that King Conan is dead. The rest of the novel revolves around Conan trying to win back his throne and overthrow his usurper.
 * In the book Hit or Myth, Skeeve has to disguise himself as King Roderick for a day so the king can take a vacation. Then the king does a bunk, leaving Skeeve to carry on the charade... which includes marrying the king's fiancée...
 * Older Than Feudalism: In The Iliad, when Achilles refuses to fight, his friend (and possible lover) Patroclus dons his armor and fights the Trojans and their many allies in his stead. Until he's killed, then Achilles is mega-pissed. However, Achilles is the protagonist while Patroclus is a side character.
 * There's a rather strange, but sad, example on Catch 22 -- while in hospital, main character Yossarian is roped in to play a dying soldier visited by his family, because the real soldier is already dead and the doctor doesn't want to disappoint them. Made especially weird by the fact that Yossarian insists they call him by his real name, rather than the dead soldier's name, and manages to convince the family (except the mother) that the soldier's name was Yossarian all along.
 * In Mary Stewart's The Ivy Tree, a young woman just come to town is recruited to impersonate the long-missing heir of a local estate, so as to convince the missing woman's grandfather to change his will.
 * In Keith Laumer's Worlds of the Imperium, the protagonist is dragooned by agents from the Imperium's Alternate History to replace a dictator (the version of himself from a third Alternate History).
 * In The Lies of Locke Lamora, the titular thief is hired by to impersonate him at a meeting with the local crime lord. He assures Locke that no harm will come to him.
 * The Mad King by Edgar Rice Burroughs is very similar to The Prisoner of Zenda except that at the end.
 * Near the end of Memory Sorrow and Thorn, the heroic army is Storming the Castle of the villains, but Prince Josua and Sir Camaris unexpectedly end up sneaking in via the tunnels -- in Camaris' case because he is compelled by the Great Sword he is carrying and Josua because he wants to try to persuade Camaris to return (and another, more personal reason). This leaves the army without its two main leaders, so the remaining ones quickly grab two lookalikes, dress them appropriately, and send them off to lead instead.
 * In the second Codex Alera novel Max winds up having to impersonate the First Lord using watercrafting after he collapses from exhaustion so the First Lord's enemies won't realize he's incapacitated.

Live Action TV

 * Probably one of the weirdest versions of this trope is in the Star Trek the Next Generation episode "The Host", which is the introductory episode of the Trill. The Trill ambassador is killed in an attack, but his symbiote is still alive. However, no other Trill is able to make it before it dies, so Commander Riker has the symbiote implanted in him. Thus, Riker is forced to carry out the ambassador's mission, which is a slight problem when the factions that requested the ambassador see him.
 * Hogan's Heroes does this three times. The first is when Kinch impersonates an African prince, the second when Crittendon impersonates an English traitor, and the third in reverse where the heroes help a German defector, who looks exactly like Schultz, escape by dressing him up in Schultz' uniform.
 * Played slightly straighter, only with time travel in Star Trek Deep Space Nine's "Past Tense", where Sisko's presence in the past caused the premature death of an important historical figure a few days before he was supposed to die heroically, then has to impersonate him.
 * In a later season, a character was studying Earth's history, and when he encountered an entry about the historical figure, passed comment about how closely Sisko resembled a picture of him (the picture, of course, being that of Sisko).
 * Sisko also, at one point,
 * Subverted though when at the end of the episode
 * The Get Smart episode "The King Lives?".
 * The classic Doctor Who serial The Enemy of the World is centered around the Doctor impersonating wannabe world dictator Salamander in order to uncover information that would discredit him.
 * A subplot of an earlier serial, The Crusade, involved knight William des Préaux pretending to be King Richard during an Arab ambush to save the real Richard from capture. Truth in Television, because this incident actually happened.
 * The Angel episode "Guise Will Be Guise", in which Wesley is forced to play the part of the missing Angel. (Considering that this story kicks off Wesley's years-long transformation from comic relief to tragic hero, this might be be the most successful impersonation in history.)
 * Happened occasionally on Sliders, when one or another character had to fill in for alternate-world versions of themselves.
 * In NCIS, Ducky impersonates the deceased arms dealer to try to capture La Grenouille.
 * In Buffy the Vampire Slayer, Willow impersonates the captured alternate universe Willow to get her mooks to go outside.
 * The Buffybot takes this role at the beginning of Season 6, impersonating the dead Buffy on patrols and Parent-Teacher Days.
 * In the Simon and Simon episode "Walk a Mile in My Hat", a potential client shows up looking for A.J. while he's out of town, and Rick decides to impersonate A.J.--clothing and behavior included--to get her business. A.J. comes back after the case has gotten complicated and winds up taking on Rick's persona just to shake up the opposition.
 * Done a few times on Xena: Warrior Princess, where Xena has at least two Identical Strangers who she either impersonates or impersonate her.
 * In two episodes of Hercules: The Legendary Journeys, Iolaus is required to step in for his lookalike cousin, Prince (later King) Orestes.
 * In an Airwolf episode, the pilot was recruited to fly a plane during an air show while the look-alike Soviet pilot was debriefed by the Firm to see if he actually wanted to defect. Unfortunately the Soviets got wind of it, brought his wife to threaten him, the pilot and the wife got kidnapped. . ..
 * An episode of The Jamie Kennedy Experiment had the mark told he bore a striking resemblance to a foreign prince who was too hung over to appear at a press conference, and was asked to take his place.

Newspaper Comics

 * Prickly City: Senator Kevin refuses to go see his constituents. So Winslow goes in disguise.

Theater

 * The play Lend Me a Tenor features a stage manager's assistant stepping in for a famous opera star who has supposedly committed suicide just before a performance.
 * In AA Milne's play The Ugly Duckling, the prince and princess who are supposed to marry are both so plain that they have better-looking servants stand in for them at the betrothal. (At the actual wedding they plan to be wearing full armor and a face-obscuring veil, respectively.)
 * In The Taming of the Shrew, a servant, Tranio, switches clothes with and fills in for his master, Lucentio, as part of a classic Zany Scheme. Later, he pushes the scheme further by getting another character to impersonate Lucentio's father.

Video Games

 * In Final Fantasy VI, Celes is asked to stand in for opera singer Maria, who happens to look exactly like her, because Setzer is planning to kidnap Maria.
 * This is the premise of the DS strategy-RPG Rondo of Swords. A prince is killed during an invasion, and asks his body double to stand in for him permanently. Since the double has been doing this for years already, he does a fairly convincing job, though "Serdic's" memory turn out to have some rather conspicuous holes.
 * Most of the first part of Wild Arms XF revolves around Clarissa impersonating Princess Alexia.

Webcomics

 * In Sluggy Freelance Torg is forced into impersonating Lord Torgamous, a medieval warlord who just happens to look exactly like him. It doesn't work out so well.
 * Arguably, the Torg Potter arcs are this as well, although it requires little effort on Torg's part to pull it off.

Western Animation

 * The Superman the Animated Series episode "Knight Time" uses this plot, with Batman disappearing, and Superman putting on his costume. Bane sure picked the wrong episode to return in...
 * "I'm twice as strong as before!" [Tries to beat up Superman.]
 * Also includes a hilarious scene where Superman mimics Batman's voice flawlessly, surprising Robin. When asked how, Supes replies with "Precise muscle control." It beats "Super Ventriloquism", at least.
 * Actually, that would have been an amusing Mythology Gag if you ask me.
 * In one Goofy short, "A Knight For a Day", Red-headed Squire Goofy accidentally knocks out his Blonde-Goofy (Okay, everyone's a Goofy, not sure if any of them is the "Real Goofy") right before a match, and had to take his place.
 * In a Danny Phantom episode, Tucker and Sam took turns dressing as Danny Fenton to ensure his parents of his whereabouts since the real Danny is currently stuck in ghost mode. They had to keep running away, declaring "a ghost took their face" since...well, his parents would probably notice that they're respectively a different race and gender than their actual son.
 * In a Phineas and Ferb episode, their dad accidentally gets into Perry's secret base and has his memory wiped. Perry has to stand in for him at Candace's Father/Daughter Competition in a robot version of him that Carl had made "just for this type of situation."
 * In the Batman the Brave And The Bold episode "Night of the Batmen," Green Arrow, Aquaman, Captain Marvel and Plastic Man all independently decide to fill in for an injured Batman. In this case, it's the impersonators that learn the Aesop, that Batman doesn't need anyone's help, because he's Batman. Even working together, they end up being captured, and Batman has to build a robotic suit and rescue them while he's still recovering.

Real Life

 * Canadian Member of Parliament Rahim Jaffer was impersonated on a radio phone-in show by an aide in March 2001. He was apparently double booked. And the aide sounds nothing like him.