The gradual distortion or even disintegration of a world and its characters during its odyssey from original source material to movie to TV movie then to television series then to video game and finally to licensed derivative work. The dramatic equivalent of photocopying a photocopy of a photocopy. The adaptation need not be subjectively worse than the original. It need only be different.
Every step away from the original property involves new input from multiple directions which dilutes and changes the flavor and behavior of the story. Adaptation Decay can sometimes be minimized, and each generation of the process will remain reasonably faithful to the original. On the other hand, a strong example of the trope would result in an In Name Only adaptation, for better or worse.
Anime frequently features some degree of Adaptation Decay, since many series are based on either manga or video games, which are subject to less censorship than TV shows. More egregious examples include dropping or adding characters. Additionally, when an anime series is brought to the United States, it may suffer further decay if it is being translated with an eye toward broadcast markets — dialogue may be arbitrarily changed or censored, and entire plotlines may be removed.
Distantly related to sequelitis. They Just Didn't Care is most of the times related to this.
Note: if you see an example of this on a work's YMMV page, please remove it. If it is in-universe example, then please move it to the main page.
Examples of fictional references to Adaptation Decay:
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Anime and Manga
In Franken Fran, a movie was made based on a previous patient of Fran's, a dog whose brain got put into a human's body. Among other things, the dog was now a pretty boy instead of an ugly balding man, the dog's owner was turned from a little girl to a young woman, Little Miss Badass Veronica was turned into The Big Guy, the dog and his owner have sex, and the story was given a happy ending where the woman's leukemia was mysteriously cured by The Power of Love. Compare that to the real events, where the girl died of pleurisy and the dog waited for her at the hospital until his own death. Veronica ends up throwing a chair at the screen and declaring the movie crap.
Deliberately done in Excel♥Saga. The anime was never supposed to stay very true to the original manga, which is far more tame than the hilarious monstrosity that is the anime. While the manga focused on mocking the recession Japan was going through at the time while using quirky humor, the anime focused on all-out wackiness using the same characters and setting. This trope is used as a Running Gag by the anime, where, Once an Episode, the Author Avatar of the original author Koshi Rikdo is forced in increasingly violent ways to give his approval to transform his creation into a different genre entirely, creating an anime that cheerfully mocks every genre it touches.
In an episode of the Buu Saga of Dragon Ball Z, a movie representing Cell's defeat at the hands of Mr. Satan is shown before the tournament. However, not only is it untrue (It was actually the Z Fighters who beat Cell), but it is embarassingly ridiculous. Only Goku was entertained, and even then, only because he considers it So Bad, It's Good.
In The Daughter of Twenty Faces, Shunka approaches one of her friends to draw a manga based off the adventures of the Bishoujo Detective Girls (a group she is a part of). Upon reading the finished product, Shunka is annoyed to find that the artist has inserted a fictitious Love Triangle in order to make the story more appealing to publishers.
An old MAD strip showed a writer submitting a film script about a teenage boy who gets into trouble with the law but learns better. At the end of a ludicrous series of rewrites at the behest of various studio execs, his script has morphed into "Cinderella".
The North Remembers has Arya Stark watching a Braavosi play about the events in King's Landing which led to the War of the Five Kings. The play itself exhibits blatant Black and White Morality (with its own Deliberate Values Dissonance), where Jaime is a comic-relief character who becomes religious at the end, and Ned Stark is executed not because he discovered the illegitimacy of Cersei's children, but because he considered Cersei too weak to rule because she was a woman. At the end, the Lannisters are either dead or publicly humiliated.
Towards the end of Scott Pilgrim vs. the World, the supporting character Michael Comeau says "The comic book is always better than the movie". He can also be overheard saying that the first album is never as good as "the first" album. Comeau's a satire of the people who are into a given social scene for the social aspect, rather than caring about the subject that supposedly unites everyone there.
The Japanese film Welcome Back, Mr. McDonald is about the live production of a radio play. Problems with the actors, sound effects, and advertisers result in them deviating from the script to the point that they are completely rewriting it every commercial break, much to the original author's distress. Needless to say, the final product ends up having very little in common with her original story.
In The Front, a gas-chamber scene has to be rewritten to use some other method of execution because the show's sponsor is a gas company.
The 1986 Alan Alda comedy Sweet Liberty is about a history professor struggling with a film adaptation of a book he wrote on the Revolutionary War.
In Neil Gaiman's short story "The Goldfish Pool", a young writer struggles to adapt one of his novels to film. Due to Executive Meddling, he is forced to discard the title, plot, characters, themes and even genre of his original book; ultimately changing it from a psychological horror story into a romantic comedy.
The Agatha Christie novel Mrs McGinty's Dead includes a playwright adapting one of Author Avatar Ariadne Oliver's books about Sven Hjerson, an elderly, Finnish, vegetarian, Celibate Hero detective. The playwright doesn't like the idea of him being a vegetarian, thinks he has to have a love interest, which means he can't be elderly, and decides he isn't even a Finn any longer, he's in the Norwegian resistance.
Briefly seen in the Doctor Who New Adventures novel Lucifer Rising, when Bernice Summerfield sees the end of a 22nd century holo-drama in which a beautiful computer expert defeats the Martians and claims the handsome museum curator. She is surprised to realise that this is meant to be the Martian invasion of 2090... just as the readers are surprised to realise it's meant to be 1969 serial "The Seeds of Death", in which the computer expert is The Spock and the museum curator is an elderly eccentric. And there's apparently no mention of the Doctor's involvement either.
The various film adaptations of Dracula in Anno Dracula: Johnny Alucard take massive liberties with the source text (which itself makes a massive deviation from what "really" happened in the Anno Dracula universe). The title film in "Francis Ford Coppolla's Dracula" was made instead of Apocalypse Now and is simply that film with the names and locations changed. In "The Other Side of Midnight", John wants as many versions of Dracula out there as possible as part of a Post Modern Magic ritual to get everyone to Clap Your Hands If You Believe in the Count; it doesn't matter if most of them are any good or not.
Live Action TV
An episode of Two and a Half Men involves a double example of this: Jake feigns having read Lord of the Flies (his book report assignment), while Charlie feigns having read the Oshikuru comic book (the Animated Adaptation of which he's writing the theme song). In the Oh, Cisco! moment, after a montage of Jake trying to help Charlie understand his source material, we get to watch Alan, Jake, and Charlie watch the premiere of Oshikuru. The show uses the exact same theme Charlie had originally written with a Lighter and Softer twist. Jake voices his incredulity, while Charlie simply says, "The network liked it."
An episode of Blossom once demonstrated almost instant Adaptation Decay in action: Nick once had a chance to pitch a TV show concept about his family life to a pair of network execs. The concept was called "Rosie", and was essentially a recursive version of Blossom. By the time the network execs got done with it, though, "Rosie" had transformed from a gentle family comedy to a detective show starring chimpanzees.
Married... with Children also demonstrated this in an hour-long ep, where Christine Applegate's talk show went from being an edgy local cable show... into a show with practically no bite at all when a network picked it up. Example: heavy metal guitarists yelling "Sex!" replaced with nice accordionists saying "Book."
Power Rangers played around with this type in the Dino Thunder episode "Lost and Found in Translation". In it, the Rangers discover a Japanese television show which seems to be based off of their adventures (which is really an episode of Bakuryuu Sentai Abaranger, the show used to create Dino Thunder). Conner is initially upset with the show for, in his opinion, making a mockery of both the Rangers and America, but by the end of the episode he learns An Aesop about diversity.
Mercilessly parodied in the Stargate SG-1 episodes featuring the Show Within a ShowWormhole X-Treme!, a television program based loosely on the "actual" events of the Stargate program which is allowed to go ahead by the powers-that-be in order to act as a cover for the real thing. Each episode that features Wormhole takes liberties with the original source material for laughs, and hangs numerous lampshades on various plot holes and inconsistencies in previous SG-1 episodes.
The X-Files episode "Hollywood AD" centred around a film being made based on Mulder and Scully's work. The Big Bad is an insane bishop using a magical artifact to take over the world, his henchmen are gun-toting zombies, Mulder (played by Garry Shandling!) cracks cheesy one-liners during fight scenes and there is a romantic subplot between the two agents.
In the iCarly episode "iCarly Saves TV", the trio are offered the opportunity to turn their webshow into a syndicated television series. By the end of the episode, Freddy had been replaced by a zany mascot, the Deadpan Snarker sidekick was replaced by an obnoxious child star, and when Carly quit, she was replaced by a sitcom family, and they changed the title. Yet the network considered it the same show, despite not even being In Name Only.
The basic plot of Episodes. Husband and wife team Sean and Beverly Lincoln are the creators of Lyman's Boys, an award-winning Brit Com about a Boarding School headmaster, and agree to make a Transatlantic Equivalent providing Richard Griffiths still stars, and nothing gets changed. The US version turns out to star Matt LeBlanc as a hockey coach, and is called Pucks!
Seinfeld almost avoided this when Jerry and George got their sitcom pilot based on themselves, but they didn't get the green light until they added the guy-sentenced-to-be-his-butler-because-he-had-no-auto-insurance, so Jerry has an indentured butler.
Inspector Spacetime (not the other one) gets an American adaptation which, courtesy of Pierce, is so bad he bypasses the Heroic BSOD and goes straight for the hate.
According to Les Luthiers, the Las Majas Del Bergant? zarzuela (a Savoy Opera about a Spanish ship whose crew is attacked by pirates) is based on a novel... about a Bulgarian lumberjack and his parrot. The only character left from the original novel was the parrot. This is forgivable, considering that Les Luthiers are a comical group and Las Majas del Bergant? is one of their most hilarious performances.
Played for laughs in Paper Mario: The Thousand-Year Door, with the Super Luigi series of books. According to them, Luigi went on a grand and epic adventure that rivals the one Mario is going on. By asking Luigi's various sidekicks, you find out that they're actually a lot less impressive than they're made out to be.
In Homestar Runner, the DVD-exclusive Strong Bad Email "Comic Book Movie" had Strong Bad describe how Hollywood would handle a movie of his comic book character Strong Badman: badly.
Zuko: That... wasn't a good play. Aang: I'll say. Katara: No kidding. Suki: Horrible. Toph: You said it. Sokka: But the effects were decent.
Prince Zuko also explains that he dislikes the Ember Island Players because of how they've "butchered" Love Amongst The Dragons.
Polly Pocket and her friends once went to the movies to watch what they expected to be the best movie ever because it was adapted from what they considered the best book ever. In the end, they considered it the worst movie ever.