The rivalry between Plankton and Mr. Krabs has long been known in the series. However, the tension between the two characters can vary in different episodes. Even though Plankton and Krabs are enemies, they sometimes show kindness towards one another. Other times, they have been downright cruel. This article covers a closer examination of Krabs and Plankton's relationship.
As shown in "Friend or Foe," Mr. Krabs and Plankton started as best friends since they were both born in the same hospital on the same day. During their elementary school years, both Plankton and Mr. Krabs were frequently bullied by other cruel children led by class bully Billy. When their favorite hang-out spot, Stinky Burgers, was shut down due to health violations, Plankton and Mr. Krabs decided to open their burger joint to earn respect from the kids.
After creating their first burger, they got Old Man Jenkins to try the first bite. Unfortunately, Jenkins died from eating the food. This sparked in a fight between Mr. Krabs and Plankton, stating that one of them did something wrong. Their fight caused the recipe to be ripped in half and Plankton's half read "a pinch of chum." After Plankton left, he slammed the door, which caused various food items to fall off a shelf and into the patty batter, giving Mr. Krabs the perfect recipe.
This incident caused Mr. Krabs to be adored by the other people for his new food sensation, while Plankton swore vengeance and attempted to steal the formula. The two have been at war ever since.
Although Mr. Krabs and Plankton despise each other, they still seem to show sympathy for each other now and then. In "New Leaf," Mr. Krabs feels heartbroken upon apparently believing that Plankton had changed. The two later make amends and go to a carnival, where Mr. Krabs eventually gives Plankton the Krabby Patty formula after accepting his change. Sadly, it is revealed that it was merely another plan by Plankton, though he did not get the formula as Mr. Krabs had tricked him with an even bigger scheme.
Another instance is in "Best Frenemies," where Plankton and Mr. Krabs team up to steal a Kelp Shake. They ultimately succeed, though they are affected by the shake's effect from drinking it. A playful side of their rivalry is shown when Mr. Krabs gives Plankton a ten-second head start in taking his formula, following a "friendly" chase.
In "Plankton Paranoia," Plankton throws a surprise party for Mr. Krabs. The party is the anniversary of when Plankton tried to steal the formula for the Krabby Patty.
The fights between Mr. Krabs and Plankton sometimes get very unpleasant in certain situations, usually when one of them intends to kill the other. The most extreme example is in "One Coarse Meal," where Mr. Krabs - upon discovering Plankton's fear of whales - dresses up as Pearl to scare him for fun. The scaring continued for 17 days, making Plankton refuse to come outside and have nightmares. Eventually, Plankton decides to lie on the road and wait for a bus to kill him. Krabs also does not care that Plankton wanted to kill himself.
Mr. Krabs tortures Plankton in "The Krusty Slammer," though by the end Plankton returns the favor by torturing him.
Plankton gloats at Mr. Krabs' misery in "Karen's Baby" after his son withholds his money.
In "Married to Money," Plankton creates a robot girlfriend for Mr. Krabs named Cashina, but by the end of the episode, Mr. Krabs finds out that it was him and is very heartbroken, telling Plankton that he was alone for so long and calls him out for exploiting that. Plankton remains completely remorseless.
The other most extreme example is in The SpongeBob SquarePants Movie, where Plankton steals King Neptune's crown and frames Mr. Krabs. This causes King Neptune to freeze Mr. Krabs and nearly kill him, only to be saved by SpongeBob and Patrick bringing the crown back from Shell City. Not only did Plankton finally steal the formula, but he also brainwashed everyone in Bikini Bottom, took over the town, and sent an evil mercenary named Dennis to kill SpongeBob and Patrick.
And other most extreme example is in The SpongeBob Movie: Sponge Out of Water where it happens backwards since Mr. Krabs turns the whole city against Plankton (and later also against SpongeBob when he believes that he betrayed him in favor of Plankton), when he believes he finally stole the secreted Formula, and in retaliation Mr. Krabs takes his restaurant, kidnapping and interrogating his wife Karen and threatening to crush him to death if he did not say where was the formula, all this despite the fact that he claimed that he did not steal it and having SpongeBob as a witness.