Sheldon J.Plankton, more commonly known as Plankton, is one of the ten main characters of the SpongeBob SquarePants franchise. He is a planktonic copepod who runs the Chum Bucket restaurant alongside Karen, a waterproof computer who is his sidekick and wife. The Chum Bucket is highly unpopular in Bikini Bottom because it serves chum, a type of bait made from fish meat.
Both Plankton and Karen are the two main villains of the franchise. Plankton despises his rival, Mr. Krabs, for running a successful restaurant because his own business rarely gets any customers. He is incredibly jealous of Mr. Krabs and his success. His main goal is to steal Krabs' secret formula for Krabby Patties and running Mr. Krabs out of business. Karen gives him a variety of plans to steal it, but their efforts always end up failing in the process.
Plankton is voiced by Mr. Lawrence, one of the show's staff writers. Stephen Hillenburg originally planned for Plankton and Karen to be one-time characters, but Lawrence wrote them into many episodes and proved that they could work as the main villains. Both Plankton and Karen first appear in the episode "Plankton!"
Plankton succeeds at stealing the formula in The SpongeBob SquarePants Movie, but his goal of world domination is eventually thwarted by SpongeBob. In "Friend or Foe," it is revealed that Mr. Krabs and Plankton were best friends during their childhood, but a dispute over the Krabby Patty secret formula ended their friendship for real, meaning that Plankton had a part in the creation of the Krabby Patty.
Plankton is, as his name suggests, a tiny green planktonic copepod who can fit in the palm of one's hand. He has some stubby appendages and one yellow eye with a red pupil. He has two long, skinny antennae with four small green spikes sticking out along them. He also has one black thick eyebrow. He has no ears or nose. In some episodes, he is shown without his skin, like in "Plankton's Regular" and "Single Cell Anniversary." His size constantly changes: in some episodes, he's the size of a Krabby Patty, meanwhile, in others, he can perfectly hide in a Krabby Patty. He also has no fingers or toes, but he can sprout four.
Plankton and Krabs were considered outcasts by other kids, most notably a school group led by Incidental K1 because Krabs was poor and Plankton was already very intelligent, which earned him the image of a nerd. When they discovered that money could bring them out of their poverty, they decided to make money selling hamburger patties and set up a mini-restaurant in the local dump.
At first, their burgers were not successful; the first of their patties were tried by Old Man Jenkins, who was knocked out by the contaminated burger. Plankton started complaining about how Jenkins had been old and not able to handle the patty. Offended by this, Krabs tried to take the recipe from Plankton to improve upon. Plankton wanted to do this himself and both he and Krabs argued, but he only ended up with the corner of the recipe, which read: "And a pinch of chum."
Plankton stormed out of the restaurant and went to create his recipe, using only his memory, the corner of the recipe, and science. At school, Plankton tried to sell his chumburgers in a chum bucket and failed, while Krabs succeeded with his Krabby Patties, which he had created by accident when Plankton had forcefully slammed the door. Ingredients fell in the patty batter, thus creating the patty. Plankton later named his restaurant the Chum Bucket, after the secret ingredient of the original unsuccessful patty recipe. Mr. Krabs named his restaurant the Krusty Krab, after himself.
To this very day, Plankton has been single-mindedly obsessed with obtaining the Krabby Patty formula and running the Krusty Krab out of business. He often goes to incredible and quite ridiculous lengths to do so. In "SpongeBob's Last Stand," for example, he builds a massive highway that goes straight through Jellyfish Fields and over the Krusty Krab, leading right to his restaurant.
Even in the present day of the series, Eugene and Plankton are occasionally shown to get along somewhat, such as in "New Leaf" and "Best Frenemies," though these moments of mutuality tend not to last very long.
The major on Plankton's degree is Tank Driving and is seen driving a tank in "Teacher's Pests" as well. 
Later, when Plankton was an adult, he married Karen. In "Girls' Night Out," Karen says that she and Plankton have been married for 26 years. Plankton is very ashamed of his first name, Sheldon, and gets embarrassed when Karen makes fun of it in "Plankton's Army." However, she calls him Sheldon in many other episodes and he does not seem to mind. It is revealed in "Walking the Plankton," that even though he's married to Karen, they never had a honeymoon.
In The SpongeBob SquarePants Movie, Plankton has exhausted all his diabolical plans and was forced to resort to "Plan Z." As per its instructions, he stole the crown of King Neptune and sold it to Shell City, framing Mr. Krabs for the deed. With Krabs cryogenically frozen by Neptune, Plankton took the Krabby Patty secret formula and became highly successful from producing the burgers. Using mind-controlling bucket helmets, Plankton turned Bikini Bottom into a Stalin-like dystopia named Planktopolis, where everyone serves the minuscule menace. However, he was suddenly stopped by SpongeBob and Patrick, who destroyed the mind-controlling helmets, thus freeing the Bikini Bottomites, and returned Neptune's crown to its rightful owner. Plankton was left crippled from being stepped on by the townsfolk, who incarcerated him in the Institution for the Criminally Tiny.
Plankton is voiced by one of the show's staff writers, Doug Lawrence, who is always credited as "Mr. Lawrence." He also voices other characters on the show; most notably, since season 10, he has been the replacement voice of Potty the Parrot. Lawrence improvises some of his dialog with his co-star Jill Talley, who plays Karen Plankton. He called the improv with Talley one of his "favorite part[s] of the voice over" in 2009. Lawrence also said in 2012: "I always enjoy the back-and-forth. [Talley and I] start to overlap so much talking to each other that [the voice directors] have to tell us, 'hey, stop doing that, separate what you're saying!'"
When the "Plankton!" episode was pitched to Nickelodeon, the series' creator Stephen Hillenburg had already decided that he wanted Lawrence to play Plankton. However, network executives wanted a guest star to provide his voice since Plankton was only set to appear in only one episode. Lawrence mentioned in a podcast interview that the studio said, in his words, "' We could stunt-cast this. You know, we could have Bruce Willis do this voice.' And Steve [Hillenburg] was just like, 'it's Doug, don't you hear it? This is the character!'"
Plankton is egotistical, aggressive, and power-hungry. He is very jealous of Krabs and his success. His only goal throughout the series is stealing the Krabby Patty secret formula and running Krabs out of business. A desire that has led him to destroy The Krusty Krab on more than one occasion. He is manipulative, as he manipulated SpongeBob into becoming "Assertive" in "Walking Small" to drive everyone in the beach away so he could build another Chum Bucket. He also manipulated SpongeBob in "F.U.N." by pretending to be his friend to get a Krabby Patty. He is also deceitful, as he is always trying to convince SpongeBob and the others that he can be trusted, only to use their goodwill to get closer to the formula. Either way, he's too busy plotting revenge to interact with most of the characters. Being a villain might be lonely, but Plankton seems to like it that way.
He also has an obsession with "world domination" as he is always mentioning that he will rule the world once he steals the Krabby Patty formula and becomes more successful. He teamed up with Man Ray in "Super Evil Aquatic Villain Team Up is Go!" and claimed he will rule the world with him, and in The SpongeBob SquarePants Movie he enslaved the entirety of Bikini Bottom and created a dictatorship.
He has a minuscule size and overcompensates by being evil and manipulative. He has tried everything from constructing an evil robot to hiring an assassin to kill SpongeBob and Patrick, but all his plans end up failing in the long run.
Plankton's restaurant, the Chum Bucket, has always been extremely unsuccessful, not only because of competition from the Krusty Krab and Plankton's rude and cruel treatment of the people in Bikini Bottom but also due to the extremely poor quality of the food and Plankton's constant failure to clean the place up. As its name implies, the Chum Bucket serves chum-based foods, all of which are shown to taste terrible. In "Plankton's Regular," the Krusty Krab adds chum to its menu in an attempt to steal Plankton's single regular customer.
The customer is disgusted by their chum and says that it is the second foulest thing he has ever tasted. This implies that chum is inherently disgusting, and cannot be made to taste good, although this is shown to be false in the episodes "Greasy Buffoons" and "Chum Fricassee." In the former, Plankton uses excess grease from the Krusty Krab's grease trap that was dumped near the Chum Bucket, to make his sales skyrocket, a goal which he succeeded in. However, his business is lost when the health inspector comes to investigate both the Krusty Krab and the Chum Bucket due to the grease trend.
It should be noted that in real life, chum is made of grounded-up fish parts that are inedible to humans, such as heads and guts, and is used as bait for sharks and similar creatures. This would mean that Plankton is killing fish, or at least collecting dead fish, and serving their remains to other fish, thus promoting cannibalism. This has only ever been touched upon in "The Krabby Kronicle," where SpongeBob prints a story claiming this. However, SpongeBob was being tricked by Mr. Krabs into printing absurd lies to appeal to customers, so this story was supposedly false. As mentioned in the episode "Spongicus," Plankton uses many odd things to spice up his chum.
Plankton has no regular employees, and runs the Chum Bucket entirely by himself, although he spends most of his time in his laboratory, due to having no customers. Plankton runs the restaurant only when he has potential customers, but is shown to be a failure fast-food owner. Also, when he does have customers, he uses artificially intelligent machines to cover the other positions.
Plankton is also highly intelligent and is one of, if not, the smartest citizen in Bikini Bottom, with his intellect being on par with only Sandy. Unfortunately, he often shows himself to be short-sighted and impulsive, and often lacking common sense. This is shown when all his plans of obtaining the Krabby Patty formula often fail, usually due to oversights, and he doesn't think beforehand of what might go wrong. He also seems unaware the biggest barrier to his success is his insistence on serving food nobody wants to eat, rather than serving something appetizing.
Despite being a villain, Plankton is shown to have a soft side, as he seems to care deeply about his wife Karen, his pet Spot and his son Chip. In "Plankton's Pet," Plankton adopts an amoeba which he named "Spot" and takes the time to train it, take care of it, etc. and was genuinely sad when it ran away, only to be reunited with it later thanks to SpongeBob.
Deception: Plankton has engaged in many unusual acts of kindness in the name of personal gain:
In "F.U.N.," Plankton pretends to be in a friendship with SpongeBob to obtain the formula from him.
In "Walking Small," Plankton deceives SpongeBob in believing that berating the beach attendants is done with kindness so he can make an unauthorized development to the property.
In The SpongeBob SquarePants Movie, Plankton acts disloyally to the king of Bikini Bottom by stealing his crown and eventually enslaving him with a mind-control helmet. Additionally, he betrays the town of Bikini Bottom by renaming it to suit himself and making the city revolve purely around him.
Taking advantage of others: When Plankton enslaves Bikini Bottom in the first movie, he forces Gary to pull a several-ton block of stone. In "Walking Small" Plankton uses SpongeBob for unauthorized land development and manipulates him into following his orders, deceiving him in believing that they are acts of kindness.
The need for power: As well as wanting the Krabby Patty secret formula, Plankton had a personality based on wanting power over others.
In the video game Battle for Bikini Bottom, Plankton creates a spade of robots to vandalize and threaten Bikini Bottom to increase his possibility of stealing the formula.
In The SpongeBob SquarePants Movie, Plankton brainwashes the citizens into worshiping him; once SpongeBob frees all the citizens from the bucket helmets, the police discover his motive and deal with him.
In "Chum Caverns," Plankton enslaves a group of cave dwellers.
In "Jailbreak!" his chum is used by criminals as knockout gas and disguises, both for robbery.
In The SpongeBob SquarePants Movie, to prevent SpongeBob and Patrick from making it to Shell City, Plankton hires an agent by the name of Dennis to deal with the duo. Dennis is eventually hit by a boat.
In "Spongicus," Plankton arranges for Patrick to be eaten by a group of lions for the entertainment of others.
Getting what he wants no matter what: Plankton breaks into SpongeBob's house in "Plankton!" to enter his brain. He also invades Patrick's house in "Salsa Imbecilicus" to get a copy of his DNA.
Disturbing the peace: In "Sweet and Sour Squid," Plankton is arrested for playing clarinet that caused a disturbance throughout the town, even though it is Squidward's fault.
Plankton is married to Karen, a waterproof supercomputer. Plankton built Karen himself out of a calculator and a mass of wires. She was originally a simple security system, as revealed in "Friend or Foe," but has become more advanced as Plankton upgrades her and his other inventions with new technology. Karen lives in the Chum Bucket laboratory and is the source of his evil plans to steal the Krabby Patty formula.
In season 12, Plankton and Karen finally have a child: a son named Chip Plankton II. Plankton initially wants nothing to do with Chip, but he changes his mind after Chip saves him. Chip later grows up to serve as an ATM at the bank. His grandmother Lily Plankton appears in "Gramma's Secret Recipe" and "Grandmum's the Word." Through an experiment, Plankton is also a DNA donor of the combined clone PlanKrab. Being a microscopic organism, Plankton's biological family numbers are in the thousands. In "Plankton's Army," he recruits his entire family to help him steal the formula, thinking them all to be criminal masterminds like himself. However, much to his surprise, his family consists entirely of uneducated, red-necked hillbillies. These relatives include, but are not limited to: Clem, Zeke, Rufus, Jeke, Billy Bob, Billy Jim, Billy Billy Bo Billy Banana Fana Fo Filly, Doug Plankton, Enis, and Rainchild, who are the only ones named.
Plankton and Eugene have a long history of a feud over the Krusty Krab. When they were kids they were friends until they had problems over who invented the secret formula as shown in the episode "Friend or Foe." Plankton often makes schemes to steal them but always ends up losing. Though they are enemies, Plankton and Mr. Krabs work together occasionally, most noticeably in "Best Frenemies" and "The Other Patty." This proves that they most likely have a on & off rivalry.
Karen and Plankton are a married couple. "Friend or Foe" reveals that they started dating while Plankton was still in school. As stated in "Girls' Night Out," they have been married for 26 years. Because they have been married so long, they are prone to bickering and starting fights. Karen is the usual source of Plankton's evil plans to steal the formula, but he often takes credit for her ideas.
Plankton and Squidward barely interact because Squidward doesn't care about the secret formula. When they do interact they don't like each other, though they have worked together on a few occasions, most notably in "Chum Fricassee" and "Sweet and Sour Squid."
Plankton and Patrick have interacted a few times, but Plankton often dislikes Patrick due to his stupidity. In the episode "Chum Bucket Supreme" however, Patrick gives Plankton a slogan that makes his place boom in popularity, so he treats Patrick well in that episode.
In SuperSponge, Plankton has a brief cameo, in which he yells at SpongeBob for stealing his time machine: "Blast you, SpongeBrat! You have found my secret time machine! Well, whatever you do, make sure you fill it up before you bring it back!"
In Battle for Bikini Bottom and Battle for Bikini Bottom - Rehydrated, Plankton devises a plan to take over Bikini Bottom and obtain the Krabby Patty formula by creating a robot machine called the Duplicatotron 3000. However, the robots do not obey him and began mindlessly rampaging across Bikini Bottom because he forgot to set the switch from "Do Not Obey" to "Obey." Plankton convinces SpongeBob and friends to defeat the robots so that he can have enough time to fix the main Duplicatotron 3000 to take over the city by controlling the robot army.
In Lights, Camera, Pants!, Plankton appears as a playable character. In the game, he uses a robotic suit to act in the auditions. Also, the "Captive" role, as well as the auditions for it, take place in the Chum Bucket.
In Nicktoons Unite!, Plankton is one of the four main villains of the game. He uses a crab mech to fight the heroes.
In Creature from the Krusty Krab, Plankton is one of three playable characters, as well as the final boss of the game. He enlarges a Krabby Patty to massive size, only to have it come to life and began chasing him. Eventually, Plankton enlarges himself to gigantic size and began rampaging across the city in pursuit of the giant patty. He is eventually defeated and reverted to his original size by Patrick, who was playing as his alter-ego Starfishman.
This list is incomplete, but you can help Encyclopedia SpongeBobia by expanding it.
Plankton is highly intelligent and has invented numerous things throughout the series:
Karen Plankton: Plankton built Karen himself from a calculator and a mass of wires. She is a waterproof supercomputer with many talents, like analyzing anything. She can also create holographic food. she was Mr. Krabs' and Plankton's security system when they were co-running their first restaurant.
In the episode "One Coarse Meal," it is revealed that Plankton is afraid of whales, enough to go insane and attempt to commit suicide. The reason is that a pod of whales once ate his ancestors. He also fears his plans failing many times.
Although absent from the final product, Plankton appears in a deleted scene from "Sandy's Rocket," where he is captured by SpongeBob and Patrick when trying to sneak into the Krusty Krab. In a chapter book adaptation of the episode, which was based on the original script, this scene is used. 
There is a running gag in many episodes he appears in where he is squished by a person.
Another running gag is him being projected through the air back to the Chum Bucket, typically by Mr. Krabs, often screaming a sentence or a prolonged word meanwhile.
It is revealed in "Single Cell Anniversary" that Plankton is not green; his outfit is. Karen removes the green and the viewer sees an underwear-clad pink Plankton embarrassed by his nakedness. He also has a rack with identical green suits.
In "Fear of a Krabby Patty," it is revealed that Plankton has a phone.
Plankton and Mr. Krabs were born exactly on the same date, as revealed in "Friend or Foe."
Out of his whole family, he is the smartest, as the rest of his family is shown to be clueless and unintelligent.
Plankton's family is made up of hillbillies, and he appears to have forgotten their stereotype after all these years, as in their episode, he believes they are "criminal masterminds" but when he sees them, he is shocked and says "I've been away from home longer than I thought."
As of "Truth or Square," Plankton has made 1,003 failed attempts to steal the formula.
At two inches tall, Plankton is gigantic for his kind. The creators did this to make viewers see him on the screen.
Plankton actually should be half the size of SpongeBob, as the latter is four inches tall.
Plankton finds Mr. Krabs' life to be worse than his life when he experiences it in "The Algae's Always Greener" and "Spy Buddies." In the former episode, he found numerous reasons that make Mr. Krabs' life difficult:
SpongeBob is always asking him for a Weekly Performance Review on Tuesday and throws a minor temper tantrum if Plankton refuses. He also suffers nervous breakdowns when finding guilty of minor errors, such as using too much sauce on a Krabby Patty or giving a customer a large drink when a medium-sized one was ordered.
Squidward complains to Plankton that he does not want to work with SpongeBob. Later, when Plankton puts him in charge of fry cooking, a customer gets angry because the Krabby Patty that Squidward cooked is unappetizing.
He learns about teamwork in The SpongeBob Movie: Sponge Out of Water.
The analysis of what is inside Plankton changes. Karen's screen in "Plankton!" shows he is 1% evil and 99% hot gas. In "Plankton Gets the Boot," he is 30% mean guy, 20% spiteful monster, and 50% evil butt-head, totaling to 100% big jerk.
Plankton's phone number is 1-800-328-2486 or 1-800-EAT-CHUM according to the episode "Gary's New Toy."
Plankton once had an official SpongeBob Twitter account, at @SheldonPlankton, but it was suspended. It can still be seen at an archive.
Tweets mentioning the @SheldonPlankton account from Karen's and Mr. Krabs' Twitter accounts are still up. Examples here and here.
According to the episode "Pineapple Invasion," Plankton does not like the taste of pineapple.
Plankton's sizes drastically vary in the episodes: in some episodes, he is the size of a jelly bean (e.g. "F.U.N."); in others, he is the size of a Krabby Patty (e.g. "For Here or to Go)", and in others, it is difficult to see him without a magnifying glass (e.g. "Plankton!").
Sometimes, his size even changes within an episode. The only consistency is his being small enough to fit in the palm of one's hand.