Many of the early Hanna-Barbera series reused stories from old Tom and Jerry cartoons (understandable, since the studio was made up of former MGM artists), as well as a few Looney Tunes (some of the Warners story men wrote for HB). For example, the T&J short "Pecos Pest", about a relative of Jerry's from Texas who comes to practice for a TV appearance and uses Tom's whiskers as guitar strings, was redone as a Pixie and Dixie short. Similarly, the Bugs Bunny cartoon "Windblown Hare", in which the Three Little Pigs sell Bugs their homes just as the Big Bad Wolf arrives, was redone with Yogi Bear.
Cartoon writer David Wise (no relation to the video game composer) did this a lot:
Wise also recycled huge chunks of his script for the Transformers episode "Microbots" for the TMNT episode "Shredder and Splintered." In the former, the Autobots shrink themselves to go inside Megatron's body, in the latter, the turtles climb inside the Kaiju-sized Krang, and from that point on the two episodes feature virtually identical dialogue and action.
Especially glaring is the TMNT episode "Poor Little Rich Turtle", which is almost beat-for-beat identical to the Transformers episode "The Girl Who Loved Powerglide".
The Adventures of Sonic the Hedgehog episode "Sonic is Running" and The Adventures of Super Mario Bros. 3 episode "Princess Toadstool For President" (both DiC Entertainment cartoons produced around the same time) both involve one of the main characters running for president against the Big Bad of the series. In the end, the hero wins in a landslide victory, and it's revealed that the villainous character received only one vote (his own) because even his two flunkies voted against him.
The Alvin and the Chipmunks episode "The Brunch Club" and the Teen Titans Go! episode "Hey You, Don't Forget About Me In Your Memory" both spoof The Breakfast Club, have the characters get in trouble for something they did on accident and wondering what they could have done to prevent it from happening, and replace Simple Minds' "Don't You (Forget About Me)" with an original song unlike most homages to the film. ("What Were You Doing At The Time?" for Alvin and "Crane Kick" for Titans).
On the subject of Teen Titans Go!, the episode "Oh Yeah!" has the same plot as the Animaniacs episode "Fake", in which one of the characters convinces their friend that wrestling is fake, but the friends do not believe what they are saying.
There's also the episode "Master Detective", which is similar to the Garfield and FriendsU.S. Acres short "How Now, Stolen Cow?", in which the main character solves a mystery involving missing farm animals.
The plot of the episode "Accept the Next Proposition You Hear" is similar to another U.S. Acres short, "Fortune Kooky", in which a character thinks that what fortune cookies say can happen in real life.
Yet another segment, "No Power", has a similar plot to one of the Garfield shorts on the same show, "Five Minute Warning". In both, the characters have to go a certain period of time without doing something they like in order to get a reward.
The episode "40%, 40%, 20%" is similar to the Arthur episode "Play It Again, D.W.", in which the main characters are tired of hearing someone play their favorite song over and over and don't understand why the character who loves the song is obsessed with it, only to have the character with the obsession lose the item which contained their favorite song.
"Fish Water" is similar to the Magic Adventures of Mumfie episode "A Fishy Tale", in which a character wants to free their pet fish they won from a carnival into the ocean when it feels sad.
Archer: Season six's "Nellis" is identical to season four's "Midnight Ron". Archer is stranded in a distant city (Montreal in "Midnight Ron, Las Vegas in "Nellis") after losing all his money at a casino and has difficulties returning home due to legal troubles (drunkenly burning his passport in "Midnight Ron", being on a No Fly List and the train equivalent in "Nellis") and someone from the office must personally get him (his step-father Ron drives to Montreal in "Midnight Ron", Cheryl, Pam, Cyril, Ray and Krieger fly out in Cheryl's personal jet in "Nellis"), their vehicle is too damaged to use further following an attack (Ron's Cadillac is run off the road by mobsters trying to rob him in "Midnight Ron", Cheryl's jet is hit by a surface-to-air missile while flying too close to Area 51 and must crash-land at Nellis AFB in "Nellis"), Archer must bluff his way out of a situation (using his pistol as a Weapon for Intimidation, first against "tranny bikers" and then two hobos with switchblades in "Midnight Ron", pretending to be CIA agent Slater and threatening a Colonel with being subjected to Project MK ULTRA in "Nellis") and ultimately Archer and companions must acquire an alternate means of transport to get home (hopping on a freight train before getting a replacement Cadillac from one of Ron's dealerships in "Midnight Ron", stealing a C-130 Hercules in "Nellis"). It even has a minor scene at the beginning of Archer being accosted by someone while using a payphone to call the office.
The season 5 Arthur episode "The World Record" has almost the same plot as the Hey Arnold! episode "World Records" (made three years earlier). In both episodes, the main characters try to break a world record until they settle down on making the world's largest pizza, with the only difference being that Arthur's attempt is successful while Arnold's fails. Both episodes also have a character unsuccessfully trying to break the record for walking backwards.
"DW and Bud's Higher Purpose" has the same plot as the SpongeBob SquarePants episode "Roller Cowards", in which two friends attempt to ride a rollercoaster but must overcome the fact that they are either too afraid or too short.
"Buster the Lounge Lizard" has a strikingly similar plot to the Recess episode "Teacher's Lounge" as both episodes center around the kids fantasizing about what the teacher's lounge is really like and then trying to sneak inside.
Episodes "Gamma World" and "Code Red" both have a villain using a fictional brand of science to disfigure crowds of people, Captain America, The Wasp, and Black Panther becoming disfigured, at least one Avenger having the antidote (created by another crimefighter) shot into himself or herself, and Thor evading a transformation before engaging in a side battle with a gamma-powered monster.
"Powerless!" has some plot elements blatantly copied from Thor's movie. Namely, Thor becomes a mortal, Loki tries to kill him with Destroyer armor, Thor sacrifices himself to protect mortal companions from the Destroyer, and Thor regains his hammer and his immortality as rewards for his selflessness.
Baby Looney Tunes seems to have recycled plots from other cartoons where infants are the main characters. "Like A Duck to Water" is similar to the Muppet Babies (1984) episode, "Beach Blanket Babies", wherein one of the characters is afraid to go swimming for the first time (Baby Daffy in the former, Baby Fozzie in the latter), while "Leader of the Pack" essentially uses the same plot as the Rugrats episode, "Tommy and the Secret Club", wherein one of the characters starts their own secret club and makes their friends do certain tasks for them in order to join (Baby Daffy in the former, Angelica in the latter).
Both the Rugrats episode "Kid TV" and the Baby Looney Tunes episode "I Strain" use the same basic plot as the Muppet Babies (1984) episode "I Want My Muppet TV". In all three episodes, the television breaks, so the babies make their own TV out of a cardboard box and act out their own TV shows with it.
The classic Beany and Cecil short Beany And Cecil meets The Invisible Man is a recycled plot to The Edgar Bergen Cartoon Show (one of Bob Clampett's lost work).
Bravestarr has two episodes, "No Drums, No Trumpets" and "To Walk a Mile", that have the same plot: "a former Galactic Marshal, who has sworn off guns due to a tragic incident in his past, is looked down upon by his child. Then, said child is kidnapped by bad guys, forcing him to take up his weapon once more." Alan Oppenheimer even voiced the former Marshal character in both episodes.
The Courage the Cowardly Dog episode "Curtain of Cruelty" has an identical plot to "The Tower of Dr. Zalost"; a scientist causes the entire town of Nowhere to become miserable, just like him (cruel in the former episode and depressed in the latter), and the solution involves one of Muriel's homemade recipes (fabric softener in the former and "happy plums" in the latter). Also, Eustace is immune because of his curmudgeoness. Both episodes do have several differences though, for instance "Dr. Zalost" is a full 30-minute episode, while "Curtain of Cruelty" is a normal 15-minute short.
DuckTales (1987) and TaleSpin both on The Disney Afternoon, did this with episodes that involved confusion over what the right date was ("Allowance Day" and "The Time Bandit", respectively), which led to an impending execution. The main character(s) were saved by a pilot (Launchpad and Baloo, respectively) who scooped away the clouds to reveal what day it really was (with an eclipse and a comet, respectively), proving who was right. Baloo mentioned that he was the first pilot who had ever done something like this, despite the fact that TaleSpin came out after DuckTales (1987). (It could be argued that because TaleSpin takes place in what appears to be The '30s, Baloo would have been the first chronologically; a view taken by at least one crossover comic.) It's worth noting that "Allowance Day" and "The Time Bandit" were written by the same writers.
Peter appearing on Jeopardy ("What is diarrhea?") from "Brian: Portrait of a Dog".
The "drive-by arguments" gag from "There's Something About Paulie".
The episode "The Splendid Source" was adapted from a short story of the same name which Richard Matheson wrote in 1956. It shows in that the episode's humor is much more sedate than the norm for the show, and is almost completely devoid of cutaway jokes.
The episode "Gronkowsbees" has a very similar plot to the King of the Hill episode "New Cowboy on the Block"; both episodes involve a famous football player moving into the main characters' neighborhoods, causing trouble and acting like total jerks, but the main characters defend the new neighbors at first despite everyone else being annoyed by them, then they eventually realize how awful their new neighbors really are and scheme to get them to move out.
While it's not exact, the episode "Quagmire's Mom," in which Quagmire is put on trial for his crimes and gets off by blaming all of his behavior on his mother, is strikingly similar to the film I Accuse My Parents.
The FG episode "Roasted Guy" and the American Dad! episode "The Great Space Roaster" have almost identical beginnings, (Peter/Roger requests his friends roast him, and winds up being genuinely hurt by the results) but go in very different directions afterwards.
An episode of The Flintstones titled "Christmas Flintstone" was expanded into an hour-long special called A Flintstone Christmas in 1977. Three of the songs in "A Flintstone Christmas" were recycled from "A Christmas Story", a TV special Hanna-Barbera made in 1972.
Garfield and Friends had a case where the same cartoon was made twice in one season, but once as a segment for each part of the show, and both had a similar name. These were "The Feline Philosopher" and "The Farmyard Feline Philosopher", where a character cannot do something they're good at because of an obstacle and asks the titular character for advice. Oddly enough, the U.S. Acres half of the show also used a similar plot in "The Old Man of the Mountain", which aired in between the two Feline Philosopher episodes.
"Clash of the Titans" from Season 7 recycled the plot of "Attack of the Giant Robots" from Season 2, with both being about Garfield waking up in the wrong cartoon.
The U.S. Acres episode "The Name Game" is similar to the earlier cartoon "Bedtime Story Blues", in which the characters change a well-known fairytale into Something Completely Different from the original story.
It's probably just a coincidence, but the last part of "Arise, Serpentor, Arise"! (G.I. Joe) and the entire episode of "Atlantis Arise!" (The Transformers) have a few similarites: the villains of the series attack Washington, D.C., are defeated by the heroes, and the treacherous character voiced by Chris Latta saves his leader (receiving no gratitude for doing so). Of course, Cobra don't ally themselves with mer-creatures, and the Decepticons don't create a new leader, but even so...
The Hey Arnold! April Fools' Day episode was a 30 minute version of their previous episode "Beaned"; both episodes involve Helga faking an injury long after she's actually healed from it to have Arnold take care of her. The resolution of these two episodes are completely different, though. In "Beaned," Helga's conscience gets the better of her for taking advantage of Arnold's kindness and she fakes her 'recovery' so that he's let off the hook. In "April Fools Day," when Arnold learns that Helga's faking her injury in order to prank him, he retaliates with an audacious prank of his own before she can spring her trap. (Since "AFD" is supposedly set post-movie, the differences in outcomes for each story show a subtle change in dynamic between the two - Arnold's passivity to Helga's aggression is slowly evolving into a good-natured 'contest of equals' between the two.)
An episode of I Am Weasel used this for a Take That!; Weasel and Baboon are filming a cartoon with the Red Guy as the director, and eventually Weasel points out that in the script, you can see the part where they crossed out "Bugs" and wrote in "Buster", and again the part where Red crossed out "Buster" and wrote in "Weasel".
The KaBlam! episode, "Won't Stick to Most Dental Work" borrows a similar premise to the Tiny Toon Adventures episode, "Thirteensomething". In both episodes, one of the two hosts leaves their show after a feud to pursue their own career and have great success in that venture (Henry opens his own restaurant, Babs stars on the titular Show Within a Show). Meanwhile, the other host (June and Buster) tries unsuccessfully to get a new co-host, inevitably leading to their show's downfall. Near the end of the episode, both parties realize how much they miss each other and reunite.
Kaput & Zösky is rather fond of this, recycling not just scripts but entire episodes themselves. One episode has them try to take over a planet, only to find all of its inhabitants fleeing because it is about to be destroyed at sundown. Kaput and Zösky try to flee, only to have the planet blow up beneath them. The episode is later repackaged as a new episode, with only new dialogue used, with the plot changed to the planet, this time a popular tourist destination, becoming unpopular.
Kim Possible gives us a Father's Day episode ("Mathter & Fervant") where teenage Ron doesn't want to hang out with his father, an actuary. The Dad then has to save the day to win his son's respect. The same plot is used in American Dragon: Jake Long with Jake and his father (also an actuary.) Both series used the same writing staff and the episodes premiered within 24 hours of each other, making the borrowing all the more egregious.
In addition, Kim Possible and Doug both have an episode ("Kimitation Nation" and "Doug's En Vogue") in which the main character inspires a fashion line, but it doesn't help their popularity at school, since everyone thinks they are just copying the look to get noticed. note Although the KP episode also had mad scientists and evil clones of the the protagonist, which the latter episode lacked. It's perhaps worth noting that Kayte Kutch and Sheryl Scarborough, who wrote the former episode, were also writers on Doug.
"A Man Without A Country Club", where Hank is offered membership to Nine Rivers, an exclusive, Asian-only country club, but as a Token Minority so the club wouldn't lose a tour by Tiger Woods, is a Race Lift of the The Jeffersons episode "Tennis, Anyone?", where George is offered membership at an all-white country club whose charter might be revoked.
"Hank's On Board" is an interesting example. The plot (sans the ending) is identical to Adrift, where a group of people go swimming off a boat and forget to lower the ladder. Adrift was released almost exactly one year after "Hank's On Board" was aired, but was written years earlier and was in production when the episode would have been produced.
"Tin Pan Alley Cats" uses the same panning shot over Wackyland as "Porky in Wackyland". "Dough for the Do-Do" is a shot-for-shot colorized "Porky in Wackyland".
Friz Freleng even recycled a few of his own Looney Tunes scripts within Looney Tunes itself. For example, the basic plots of "His Bitter Half" and "Honey's Money" are the same: A money-grubbing man (Daffy Duck in the former, Yosemite Sam in the latter) marries a woman for her money, and eventually has to take care of the woman's son. They even share a scene: The shooting gallery where the son makes it seem like Daffy or Sam is shooting at the barker. "Hmm...must have rick-o-shetted!"
Not only is the show similar in premise to Stuck in the Middle, but they both have episodes where the main character tries to claim the best seat in the family car.
"Undie Pressure" is similar to the Ed, Edd n Eddy episode "All Eds Are Off" as both are about the characters entering a bet to see which of them can go without their annoying habits the longest for a reward.
"Ties That Bind" is similar to the Muppet Babies (1984) episode "Eight Take-Away One Equals Panic" and The Powerpuff Girls episode "Little Miss Interprets", all three plots revolving around some misunderstandings leading to a belief among the kids of the family (The Louds, the Muppets, and the PPG, respectively) that their guardians are going to get rid of them.
"One of the Boys" is similar to the Dexter's Laboratory episode "Oh, Brother"; both of which involve the main characters ending up living with brothers instead of sisters, only for it to be not as cool as they thought it would be.
The episode "Pipe Dreams" has a similar premise to "Serious Business" from Teen Titans Go!, in which one character is so fed up with people taking long in the bathroom that they come up with their own solution to the problem, which is different for both shows (in Titans, Robin establishes a 5-minute rule, while in Loud House, the parents build their own bathroom).
The Magic School Bus episode "Inside Ralphie" is very similar to the Muppet Babies (1984) episode "Scooter's Uncommon Cold": a "Fantastic Voyage" Plot where one of the characters gets sick, the others all shrink themselves and go into his body, and they learn about the immune system by seeing how their friend's is working to make him well. The main difference is that the Muppet Babies only imagine themselves going inside Scooter, while Ms. Frizzle and her class go inside Ralphie for real.
Martha Speaks intentionally used the same basic script for "Martha Smells" and "Martha Hears" which were part of the same episode. This is explained as T.D. copying Helen's script with some minor changes. The end of "Martha Smells" foreshadows the end of "Martha Hears". "Martha Hears" had some of the characters wondering if the same situation already happened.
The Mighty B! episode "Dogcatcher in the Rye" has the same plot as the Rocko's Modern Life episode "Ed Good, Rocko Bad". Both center on the protagonist running for dog catcher against their archenemy, only for the latter to use slander against the former to win.
In "The Ticket Master", Twilight is given two tickets to the Grand Galloping Gala. She takes one for herself but can't decide on which of her then-new friends is most deserving of the other and eventually the whole town pesters her for the ticket. An episode of My Little Pony Tales had a similar premise. In "And the Winner Is...", Clover is given two concert ball tickets and can't decide on who deserves the extra one.
"Ponyville Confidential" bears similarities to the Ed, Edd n Eddy episode "Truth or Ed" and the SpongeBob SquarePants episode "The Krabby Khronicle", where the characters get involved in a slanderous newspaper business.
"Sisterhooves Social": Rarity and Sweetie Belle get on each other's nerves to the point where Sweetie Belle disowns Rarity as a sister. The two meet again later on a camping trip Sweetie Belle is having with Applejack and Apple Bloom, but tensions remain high between them. Sweetie Belle realizes that Rarity is a sister worth having after participating in, although not winning, the Sisterhooves Social race. "Oh, Brother!": Mario and Luigi get on each other's nerves to the point where Luigi disowns Mario as a brother. The two meet again later in the middle of a rainstorm, but tensions remain high between them. Luigi realizes that Mario is a brother worth having after saving him from one of Bowser's schemes.
"Hearts and Hooves Day" is similar to The Powerpuff Girls' "Keen on Keane". It involved the trio being Shipper on Deck and ensuing disasters. The love interest is both cases is a female kindergarten teacher (Ms. Keane, Cheerilee) and an older male relative (Prof. Utonium, Big Macintosh.)
"Putting Your Hoof Down" recycles the premise of another Powerpuff Girls episode entitled "Bubblevicious". Both stories involve the most sensitive main character wanting to prove that she has a spine, and ends up going too far with her newfound confidence.
A possibly unintentional one, but "A Canterlot Wedding" is similar to parts of the South Park episode "Succubus". Both involve the protagonist(s) finding something off about their friend's fiancee and accuse her of being evil, leading to her running off in tears and the friend to call out the protagonist(s) even though (s)he/they was/were right in a way. The difference is that while the South Park boys hated Chef's fiancee Veronica from the start and she was clearly an evil monster, Twilight Sparkle's reason was more tragic because the wedding was between her older brother Shining Armor and her beloved foal-sitter Princess Cadence, who turns out to have been kidnapped by Queen Chrysalis, the ruler of the Changelings who happens to prey on Shining Armor's love by taking on Cadence's form.
"Maud Pie" is similar to the Hey Arnold! episode "Weird Cousin". Both episodes have a character's rather peculiar relative visit the main cast. Coincidentally, Pinkie Pie's sister Maud speaks in the same monotone as Arnold's cousin Arnie.
"Magical Mystery Cure" and "Crusaders of the Lost Mark" share a similar formula: both are Musical Episodes where the protagonists undergo a major transformation after solving a major cutie mark-related issue. In the former, Twilight grows wings after figuring out how to return her friends' swapped cutie marks to normal and is crowned a Princess of Equestria. In the latter, the Cutie Mark Crusaders discover they share a talent for helping others understand their own talents after helping Diamond Tiara through a mark-induced identity crisis, which leads to them earning their cutie marks.
The episode "Rarity Takes Manehattan" is very similar to the Family Ties episode "Designing Woman" where Mallory finds out a co-worker is stealing her fashion design ideas.
It also has much in common with an episode of its sister show, Littlest Pet Shop (2012), "Plane It On Rio!" In both episodes, a fashionista travels to a distant city to participate in a major event and meets an old acquaintance, who proceeds to steal and plagiarize the fashionista's ideas, and she resolves the issue by creating even better designs to defeat her unscrupulous rival. Both episodes also debuted within a month of each other.
"The Crystalling" involves the birth of Shining Armor and Princess Cadence's baby Flurry Heart. Upon birth, she involuntarily uses her magic. That aspect is similar to "Fairly Odd Baby", in which the newborn Poof has no control over his magic. Coincidentally, the baby's cry causes something bad to happen, although the danger triggered by Flurry Heart's crying was by accident.
The episodes "The Mysterious Mare-Do-Well" and "28 Pranks Later" have almost identical basic plots. In them, Rainbow Dash is acting more obnoxious than usual, so the other ponies form a plan to take her down a peg and teach her the error of her ways (inventing a superhero to upstage her at every turn and staging a Zombie Apocalypse, respectively).
"A Flurry of Emotions" is pretty much "Baby Cakes" with Twilight in Pinkie Pie's place, as she looks after her niece Flurry Heart.
The episode "Secrets and Pies" is similar to the My Friends Tigger & Pooh episode "Piglet's Thousand and One Watermelons".
The Mysticons episode "Quest of the Vexed" shares the same plot to the My Little Pony: Friendship Is Magic episode "All Bottled Up". Both centered around a character being angry at someone and they use some kind of magical object to bottle up their anger, only for the object to break and spread the anger to everyone around it.
The PAW Patrol episode "Pups Save A Stinky Flower" has a similar plot to The Doodlebops episode "The Eew Flower", where a character brings their friends a smelly flower that they think they would like, but their friends do not want it.
"Pups Save a Snow Monster" is very similar to "The Yeti" from The Backyardigans, where the cast looks for the titular Monster, and one character (In Backyard, it's Tasha, in PAW, it's Ryder) tries to convince the others that there's no monster. In the end, it turns out there actually was a monster, but it's really just their friend covered in snow. In Backyard, it's Pablo, in PAW, it's some farm animals.
The show also had two episodes ("Pups Turn On the Lights" and "Pups Save a High Flying Skye") where a Big Blackout prevents the someone from doing something that they were planning for a while (in the former, the blackout ruin's Chase's birthday, while in the latter, a farmer cannot do work because of it).
The Pinky Dinky Doo episode "Pinky Dinky Re-Doo" has the same plot as The Big Comfy Couch's "Ain't It Amazing, Gracie?". Both involve characters losing a beloved item and trying to retrace their steps to find it and end with a relative giving them the item that they lost.
Many Popeye cartoons from Famous Studios were remakes of old Fleischer Studios shorts, such as "The Anvil Chorus Girl" (based on "Shoein' Hosses") and "Penny Antics" (based on "Customers Wanted").
The plot of "Olive's Boithday Presink", especially the gag of the hunted tricking the hunter into thinking he has a family, was reused in the Looney Tunes short "Duck Soup to Nuts". Both were written by the same guy.
"Olive Oyl for President" had the exact same script as "Betty Boop for President", replacing Betty with Olive, of course.
The episode "What's The Big Idea" borrows a similar premise to the Adventures of Sonic the Hedgehog episode "Too Tall Tails", in that the villain grows one of the main characters (in The Powerpuff Girls' case, the whole trio) to gigantic size so that they cause more harm to a city than good.
The Powerpuff Girls Movie is almost an expansion of "Mr. Mojo's Rising", detailing about how the girls were created and how Mojo Jojo became who he is today.
The episode "Bubblevision" to "The Mane Event". Both have one of the sisters get an embarrassing attribute (A bad haircut for Blossom, and Nerd Glasses for Bubbles), the two other sisters laugh at her which causes her to lose confidence, but in the end, she uses the attribute to defeat the monster attacking the town.
"Strong Armed" has similarities to "Bubblevision". Both include Bubbles having a medical condition, with the former having her break her arm and the latter revealing that she is near-sighted.
"Arachno-Romance" is similar to "Mommy Dearest" in that the Professor gets a girlfriend but his daughters don't like her.
"Man Up" is almost exactly the same as "Makes Zen to Me".
The crossover with Teen Titans Go! has a similar plot to "Members Only". An older group of superheroes acts condescending towards the Powerpuff Girls, though in this case it's due to their age rather than gender. It also has elements of The Powerpuff Girls Movie with Mojo Jojo commanding an army of simians.
The Recess episode "The Hypnotist" has the same basic scenario as the Rugrats episode "Regarding Stuie": the show's most important adult male is somehow made to believe he's a child, befriends the main kid characters and has fun with them, but eventually the kids realize they need to change him back. In the Rugrats episode, Stu becomes baby "Stuie" due to Easy Amnesia from a bump on the head, but after a while Tommy misses his dad; in the Recess episode, Principal Prickly becomes six-year-old "Petie" due to a hypnotism act gone awry, but when Miss Finster takes over the school in his absence, the kids realize they want Prickly back.
Mordecai and Rigby mess up/break something and must repair it. In most of the episodes that had this plot, it'll usually end up with a "Shaggy Dog" Story. (ex: "Limousine Lunchtime", "Tent Trouble", "Garage Door")
Mordecai and/or Rigby wanting to accomplish at something, usually a video game. (ex: "High Score", "Slam Dunk", "Bank Shot", "Happy Birthday Song Contest")
Mordecai trying to impress Margaret in the first four seasons.
"Rigby's Body" has an eerily similar plot to the Aaahh!!! Real Monsters episode "Who'll Stop the Brain", in which Ickis and Krumm try to search for Oblina's brain after sneezing it out from studying for an exam to the point of exhaustion after being warned previously like Rigby when his consciousness has to be retrieved after being forcefully yanked out of his body from eating too much junk food.
The episodes "Million Dollar Abie" and "The Boys of Bummer" both involve a member of the Simpson family (Grampa and Bart respectively) becoming a pariah over a sports-related mishap, to the point they attempt suicide. Though in the former's case, it only took over the first act, whereas the latter became the episode's main dilemma.
The episodes that focus on Homer and Marge's marriage crisis, Homer getting a job, Bart getting a new girlfriend, Homer trying to be a better father, and Lisa wanting to be popular (usually when she befriends a one-time character). They're the most used plots in this show.
"Bart Sells His Soul" had Moe remake his bar into Uncle Moe's Family Feedbag to make more money. Later episodes had him remake the bar into a swanky hipster joint, and an English style restaurant, although the conflicts addressed in each episode are subtly differed, and the bar usually was only the starting point.
The subplot for "Realty Bites", where Snake tries to kill Homer when he buys his car at a police auction, was previously used in The Flintstones episode "Fred's Second Car".
In a rare instance of The Simpsons borrowing a plot from Family Guy, the "Treehouse of Horror XIV" short "Reaper Madness" involves Homer accidentally killing Death and having to take his place when he creates a world where people are unable to die, much like the Family Guy episode "Death is a Bitch" where Peter has to take Death's place after the latter twists his ankle.
The Smurfs themselves would recycle the same plot of Season 1's "The Fake Smurf", with "The Baby Smurf" (also Season 1) and "The Mr. Smurf Contest" (Season 5).
The main plot of South Park: Bigger, Longer & Uncut is an extended musical version of the subplot in "Death". Here, Kyle's mom overreacts to Terrance and Phillip and gets the other parents to protest against them, only in this case it escalates into a war between the United States and Canada.
Another episode of Spider-Man (1967), involving a scientist taking over a power plant to raise the city into the air, was re-used later. Essentially they changed a few words in the script, changed the scientist's skin color and added pointy ears, and suddenly it was involving an Atlantean using his submarine to lower the city into the ocean.
A few later episodes of SpongeBob SquarePants seem to have recycled plots from other Nicktoons. "Toy Store of Doom", for example, has essentially the same plot as the Rugrats episode "Toy Palace" (they get locked in the toy store after it closes for the night and are afraid the toys will attack them), while "Banned in Bikini Bottom" (Krabby Patties are outlawed and Mr. Krabs starts selling them at SpongeBob's house secretly) is similar to the CatDog episode "Just Say CatDog Sent Ya," in which Farburg Burger Bones are banned from Nearburg and CatDog starts selling them at a speakeasy in an underground cellar.
"Picture Day" was a recycled script from the Recess episode "One Stayed Clean". An earlier episode recycling a script from the show would be "Big Pink Loser", which was almost identical to "Copycat Kid".
"Fear of a Krabby Patty" partially recycles its plot from the earlier SpongeBob episode "Graveyard Shift". Both episodes involve Mr. Krabs changing the Krusty Krab's business hours to run for 24 hours a day, despite not having the staff necessary to do so, but for different reasons; in "Graveyard Shift" it's because he discovers that he can get more customers if the Krusty Krab was always open, and in "Fear of a Krabby Patty" it's to spite Plankton after he opens the Chum Bucket for 23 hours (setting his plan for the episode into motion). The main difference is that in "Graveyard Shift" we only see the Krusty Krab open for one night shift and in "Fear of a Krabby Patty" it's open for 43 days straight.
"Krab-Borg" borrows a similar premise to the Rugrats episode "Real or Robots?", wherein two characters believe another character is a robot after seeing a scary robot movie.
"To Love a Patty" has the same premise as The Ren & Stimpy Show episode "I Love Chicken", where a character falls in love with a food item that they were preparing for someone else.
"Born Again Krabs" and "Cuddle E. Hugs" both have their plots started by a character eating a clearly rotten Krabby Patty because they believe it's still good.
"Squilliam Returns" and "Grandmum's the Word" both involve a member of the main cast pretending to be the owner of the Krusty Krab to impress a minor character.
"Best Frenemies" and "The Other Patty" both have Mr. Krabs and Plankton forming an Enemy Mine to combat a new restaurant that threatens to put them out of business, though they're teamed up for most of the former and only begrudgingly team up near the end of the latter.
"The Paper" and "Idiot Box" both involve SpongeBob doing a lot of fun things with an ordinary object due to his imagination and then Squidward trying to do the same thing with said objects and being unable to do so.
"Nasty Patty" is quite similar to the Rocket Power episode "It Was a Dark and Stormy Day"; both episodes take place on a rainy night with the main characters assuming that someone else was killed and both episodes also mostly take place in the Local Hangout restaurant of the series (Krusty Krab/Shore Shack), and in both episodes the cops get involved and both end with an Everybody Laughs Ending with the cops and main characters hanging out in the restaurant.
"Broken Alarm" is similar to the Rocko's Modern Life episode "Commuted Sentence", where the main character keeps showing up late to work and needs to find a way to make it on time. Where Rocko ultimately loses his job at the comic book store and gets a new one as a tow truck driver, Mr. Krabs heavily implies that he'll give SpongeBob the axe, only to admit that he'll just start opening and closing the Krusty Krab an hour later to fit his schedule.
When scifi author Larry Niven was hired to write an episode of Star Trek: The Animated Series, he took the plot of his short story "The Soft Weapon" and replaced three of the characters with Enterprise crew to create "The Slaver Weapon". It even featured one of his trademark alien species, the Kzinti, without alteration. (His rejected original proposal for the episode, meanwhile, became another short story, "The Borderlands of Sol".)
The Star vs. the Forces of Evil episode "St. Olga's Reform School for Wayward Princesses" is very much like the Phineas and Ferb episode "Phineas and Ferb Get Busted!"; both involved a character trying to save loved ones from an Assimilation Academy that's stripping them of their identities. Incidentally, they had the same writer.
The entire premise of the show is very similar to the episode "Jack and the Flying Prince and Princess" of Genndy Tartakovsky's Samurai Jack. Both even feature their robot companion dying, only in Sym-Bionic Titan, said robot is revived.
Tom and Jerry: The 1970s TV episode "Stay Awake or Else..." is similar to the theatrical short "Sleepy-Time Tom", in which Tom returns home sleepy from a party and is in danger of losing his job if he's caught sleeping by his master.
Take a typical episode of Wacky Races, find a visual gag involving Dick Dastardly's attempt to stop the other racers, and the odds are even that you'll find an identical gag in a Chuck JonesRoad Runner cartoon. (Michael Maltese is credited as a writer on both series.)
The Yin Yang Yo! episode "Touchy Feelings" was about Yang having all of his emotions magically removed, much like Timmy in The Fairly OddParents! episode "Emotion Commotion". Creator Bob Boyle was a staff member on the latter series.