Plankton despises his rival Eugene Krabs for running a successful restaurant because his own restaurant, the Chum Bucket, is unsuccessful. Plankton runs the restaurant alongside his sapient computer wife Karen, who is usually the source of his evil plans.
His primary goal is to steal the Krabby Patty secret formula and put Mr. Krabs out of business. Plankton succeeds at stealing it in the 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, meaning that Plankton had a part in the creation of the Krabby Patty.
Plankton is, as his name suggests, a tiny dark green-colored plankton who can fit in the palm of one's hand. He has 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.
Sheldon J. Plankton was born as the only known child of Mama and Gordon Plankton, and is the grandson of Grandma Plankton. He was born on November 30, 1942. This is also the birth date of his former best friend and current rival, Eugene H. Krabs.
When they were kids, Plankton and Krabs were considered outcasts by other kids, most notably a school group led by Billy 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 him 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 own 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 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 simply to do so.
In "Dunces and Dragons," it is implied that the fight between the Plankton and Krabs families began centuries ago when Plankton's evil overlord ancestor, Planktonamor, was vanquished by a Krabby Patty. Since there was not any feud between Krabs and Plankton in their early childhood, it is possible that these families settled their differences until the childhoods of Krabs and Plankton.
Even in the present day of the series, Krabs and Plankton are occasionally shown to somewhat get along, such as in "New Leaf" and "Best Frenemies."
The major on Plankton's degree is Tank Driving.
Later, when Plankton was an adult, he married Karen.
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 taste terrible. In "Plankton's Regular," the Krusty Krab adds chum to its menu in an attempt to steal Plankton's single regular customer (who was in reality being paid by to eat at the Chum Bucket and pretend to like the food there, so that Plankton would "quit complaining").
The customer is disgusted by their chum, and says that it is the second foulest thing he has ever tasted (it can be inferred that Plankton's chum is the number-one worst). This implies that chum is inherently disgusting, and cannot be made to taste good, although this is shown to be false in the season 7 episode "Greasy Buffoons," and in the episode season 8 "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, although it is unknown if this made the chum taste good, as Plankton could have been lying, and his customers could have been tasting only the grease, as the flavor could have covered up the chum's flavor.
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. It is also possible that in the alternate reality the show takes place in, chum might be made of something completely different (although still likely disgusting), as mentioned in the episode "Spongicus," Plankton named off many weird things that make up his chum, and fish parts were not on the list.
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. In addition, when he does have customers, he also uses artificially intelligent machines to cover the other positions.
Although it has been stated on multiple occasions that Plankton has never had a single customer, he has had brief success on several occasions, particularly in the more recent episodes, such as "Spongicus" and "Chum Caverns." A fan once wrote to Stephen Hillenburg, questioning that, due to Plankton being a genius in the fields of robotics, inventions, and technology (including many other fields), why Plankton doesn't sell computers and the like instead of fast food (which would actually probably bring him much better success). Hillenburg answered this with saying that Plankton has never gone down this route, simply due to him having never thought of it before, and also due to the fact that Plankton is slightly incompetent, and also that he has terrible communication skills, he would probably ignore the person's idea until the last minute.
In The SpongeBob SquarePants Movie (which is confirmed to take place at the end of the series), Plankton had exhausted all of 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's "wife" is his computer system, Karen, even though neither of them fully consider the other their true soulmate, and their relationship is a very troubled one, although it was slightly mended by the events of the episode "Komputer Overload." Sometimes, Plankton treats Karen absolutely terribly, such as referring to her simply as "computer wife," and building a replacement wife, Karen 2, and expecting Karen to be okay with it in "Karen 2.0." He also constantly blames her for his failures.
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 Plankton, Doug Plankton, Enis, and Rainchild, whom are the only ones named.
Plankton and Mr. Krabs 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 help each other sometimes.
Karen is Plankton's computer wife that he built. They have troubling marriage issues, but they still love each other nonetheless. Karen often gives Plankton ideas on how to steal the formula though he just takes it his own way. There have been a few instances where they would break up with one another or Plankton leaves her for another woman. They still love each other nonetheless.
Plankton and Squidward barely interact due to the fact Squidward doesn't care about the secret formula. When they do interact they despise each other though. Though Plankton pretended to like him in the episode, "Sweet and Sour Squid."
Plankton and Patrick have interacted a few times, but Plankton often despises Patrick due to his stupidity. In the episode "Chum Bucket Supreme" however, Patrick gives Plankton a slogan that makes his place boom in popularity.
Plankton is afraid of Pearl. This is shown in the controversial episode "One Coarse Meal." However in other episodes like "The Algae's Always Greener" he doesn't mind her, he just finds her annoying and nagging.
Gary the Snail
Plankton and Gary have a strong dislike for each other. Plankton pretends to be him in "Shellback Shenanigans." And in the episode "Pineapple Invasion," Plankton and Gary fight because Plankton tries to get the formula.
Aiding in crime: In "Jailbreak!" his chum is used by criminals as knockout gas and disguises, both for robbery.
Attempted murder: In "Spongicus," Plankton arranges for Patrick to be eaten by a group of lions for the entertainment of others.
Attempted murder by proxy: In The SpongeBob SquarePants Movie, to prevent SpongeBob and Patrick from making it to Shell City, Plankton hires an assassin by the name of Dennis to kill the duo. Dennis is eventually hit by a boat, so Plankton's attempt to have SpongeBob and Patrick assassinated backfired on him.
Attempted theft: In most of his major appearances, Plankton makes an effort to obtain the Krabby Patty formula, with his plans usually backfiring on him in the end. He gets arrested twice for doing this in "Krabby Road."
Framing: In the first movie, Plankton accuses Mr. Krabs of stealing King Neptune's crown so as to get him executed and thereby finally succeed in stealing the formula.
Fraud: 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 so as 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 "New Leaf," Plankton claims he's changed his ways and showcases that through turning the Chum Bucket into a gift shop, but this was really just a way of deceiving Mr. Krabs. In "Krabby Road," Plankton forms a band with SpongeBob, Patrick, and Squidward with the intent to obtain the secret formula.
Grand theft of royal property: Plankton steals King Neptune's crown in the first movie and sells it to Shell City without his consent.
Lying under oath: In "Krabs vs. Plankton," Plankton fakes his injuries in an attempt to sue Mr. Krabs.
Manslaughter: A flashback in "Friend or Foe" reveals that Mr. Krabs and Plankton's Plabs Burger poisoned Old Man Jenkins to the point of death, however this was completely unintentional on the boys' part, as their intention was to distribute and sell pure food to the public. Nevertheless, this incident gives them instant publicity and begins their notorious reputations.
Practicing without a license: As a means of deception, Plankton has posed under many job titles without having the license required or official permission to hold such occupations; he impersonates a therapist in "Fear of a Krabby Patty," a salesman in "Shellback Shenanigans," and a Girl Scout in "Pineapple Invasion."
Prison escape: Plankton escapes from jail in the episodes "Krabby Road" and "Jailbreak!"
Proxy terrorism: 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.
Slavery: 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. In The SpongeBob SquarePants Movie, Plankton enslaves the entire population of Bikini Bottom and brainwashes the citizens into worship him; once SpongeBob frees all the citizens from the bucket helmets, the police discover his motive and incinerate him in a mental institution for the tiny. In "Chum Caverns," Plankton enslaves a group of cave dwellers.
Theft: In The SpongeBob SquarePants Movie, he finally gets a hold on the Krabby Patty formula. He also steals Sandy's fur pelt in "Someone's in the Kitchen with Sandy" and a piece of famous art in "Fiasco!" (albeit unintentionally), the latter instance of which lands him in jail with Squidward.
Treason: In the first movie, Plankton acts disloyal 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.
Violation of food safety laws: Plankton has poisoned his customers with his chum in numerous episodes, such as in "Plankton's Regular," "Spongicus," and "For Here or to Go." In "Friend or Foe," he and Mr. Krabs poisoned Old Man Jenkins with their Plabs Patties to the point of his death; however, this was done on accident, so it doesn't count on Plankton's criminal record, but it still begins the notorious reputation of the boys.
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, 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 begin 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 (as SpongeBob believed that the situation was his fault) 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 (the parts that are played). In addition, 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 begin chasing him. Eventually, Plankton enlarges himself to gigantic size, and begins 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.
Karen: Robot computer/wife that can analyze anything. She is mobile. She can also create holographic food. Apparently 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. Another reason might be because whales feed on Plankton. He also fears his plans failing many times.
""Well, we like to take long walks on the beach, and, uh, we like to talk about our dreams. Our dreams of conquering all of Bikini Bottom and become supreme overlords of all creation and to trample our enemies beneath our feet."" —"Single Cell Anniversary"
""Thanks to your brilliant AIM, honey, now I'm stuck inside that fool of a fry-cook!"" —"The Inside Job"
""There's nothing a little duct tape and surgical tubing can't accomplish. Ow! I knew I should've used pipe cleaners. Hello, I'm your long-lost relative. Can I have the secret formula?"" —"Sweet and Sour Squid"
""The customer?! I'LL BOIL THE CUSTOMER IN HOT OIL AND RIP OUT HIS- Oh yes, for the lovely customer."" —"The Algae's Always Greener"
I went to college.
""I just wanna talk."" —"[]"
""But when I add Patrick's DNA to it, it'll become Salsa Imbecilicus."" —"Salsa Imbecilicus"
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 (including the movie) 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 actually 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.
Plankton and Mr. Krabs were born exactly on the same date, as revealed in "Friend or Foe."
Interestingly, although his surname is "Plankton," he bears more of a resemblance to the real-life class of organisms, copepoda (cyclops), which feed on phytoplankton. Do note that plankton are known to come from a different kingdom than copepods.
Despite all of Plankton's schemes, he has only been arrested five times: The SpongeBob SquarePants Movie, "Krabby Road," "Sweet and Sour Squid," "Fiasco!," and "Jailbreak!" He escaped from the cops in "Goo Goo Gas."
He is called "Shelly" by Mr. Krabs in "SpongeBob's Last Stand."
Plankton is college educated, which has become a running gag throughout the series, as he would usually shout "I went to college!"
Stephen Hillenburg says that chronologically, The SpongeBob SquarePants Movie is the series' end and thus takes place after every episode (even those that have yet to be aired). However, in the movie, Plankton says he's never had a customer, but in "Chum Caverns" and "Chum Bucket Supreme," he has one at least, and usually more than his only competition, the Krusty Krab.
Plankton is afraid of whales in "One Coarse Meal," but not in "The Algae's Always Greener" and "Jailbreak!"
Out of his whole family, he seems to be the smartest, as his family is shown to be clueless and unintelligent.
Plankton's family is made up of hillbillies (or what they appear to be), 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." It is possible that after this, he was disgusted by their stupidity, and possibly taught them to be smarter.
According to the TV movie "Truth or Square," Plankton has made 1,003 failed attempts to steal the formula.
At 2 inches tall, Plankton is actually gigantic for his kind. The creators, of course, did this to make viewers see him on the screen.
Plankton actually should be half the size of SpongeBob, as the latter is 4 inches tall.
Plankton is the smallest of all the main characters. He is also the smallest citizen of Bikini Bottom.
In "Plankton!," he has a lab, which is just a real Labrador.
In later episodes, when SpongeBob holds Plankton in his hand, he is bigger or SpongeBob's hand is smaller (compare "Plankton!" with "Single Cell Anniversary" or "The Main Drain").
In "The Main Drain," Plankton is unusually bigger than he is in other episodes. This also happens in "Sweet and Sour Squid."
He and Mr. Krabs are the only members in their families to ever make friends with each other; their predecessors are enemies.
In several episodes from late-season 4 to early season 5, the inside of his mouth is purple. It was corrected back to red in later episodes.
Plankton has inventions for equipment in contrast to Mr. Krabs' spy technology.
In the episode "Sweet and Sour Squid," Plankton is revealed to have ears.
Arguably, the first movie is the only time Plankton turns truly malevolent, and that is perhaps the first film where he most reaches the summit of malignancy: He had to almost kill Mr. Krabs, enslaved all citizens of Bikini Bottom, and hired a mercenary to kill SpongeBob and Patrick.
Plankton found Mr. Krabs' life to be worse than his life (because Plankton ruins Mr. Krabs' life) when he experienced 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 found 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, the lazy, grumpy, pessimistic worker at the restaurant, complains to Plankton that he doesn't want to work with SpongeBob. Later, when Plankton put him in charge of fry cooking, a customer gets angry because the Krabby Patty that Squidward cooked is very ugly, disgusting-looking, and hideous.
Krabs tries to steal the Krabby Patty formula while naked (which Plankton actually does in the show's reality), with an annoying persistence that finally convinces Plankton that Mr. Krabs' life is "just not worth it."
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 (total of 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 was the first character in the series to have an episode with his name in the title.
He was the last main character to debut.
Plankton is the only main character who doesn't have an official SpongeBob Twitter account.
Despite this, he does have an official Facebook (see External links).
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 jellybean; in others he is the size of a Krabby Patty, and in others, it is difficult to see him without a magnifying glass.
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.