Squidward, being the employee of Mr. Krabs, is often generally considered the enemy of Plankton, who is the competitor of Mr. Krabs. Although, the two have worked together on a few occasions, most notably "Chum Fricassee."
Episodes as friends
In the episode "Mermaid Man vs. SpongeBob," after Squidward is attacked by the kids, Plankton shows sympathy for Squidward's situation.
In the episode "Sweet and Sour Squid," after SpongeBob tells Plankton that he needs to use Squidward's clarinet playing to get to Squidward, Plankton pretends to like Squidward's clarinet playing, which Squidward appreciates. Although, this instance was obviously done out of self-gain on Plankton's part.
In the episode "Walking the Plankton," Squidward is one of the audience members of Plankton's vacation slides.
In the episode "Chum Fricassee," Plankton offers Squidward a job at the Chum Bucket. Later, Squidward shows Plankton his grandma's recipe for Chum Fricassee, which allows both Squidward and Plankton to become successful.
In the episode "Company Picnic," Plankton convinces Squidward and Mr. Krabs to join his company picnic.
In the episode "Spin the Bottle," Plankton grants Squidward his wish when he is pretending to be a genie.
In the short "Leo," Squidward uses Plankton in a lion outfit as Leo.
In "Plankton Paranoia," Squidward and the gang secretly team up with Plankton to help put Mr. Krabs' mentally insane behavior to a stop because it was ruining the restaurant business. To do that, they plan an anniversary party of when Plankton first tried to steal the secret formula, which turns out to be a success.
Episodes as enemies
The first time these two were shown to have a bad relationship was in the episode "Plankton!," where Plankton, who has taken over SpongeBob's brain, insults Squidward's clarinet playing.
The next time this is shown is in "F.U.N.," where Squidward notices Plankton and informs Mr. Krabs about his presence in the restaurant.
In "The Algae's Always Greener," Squidward scolds Plankton for making SpongeBob work at the cash register with him. Plankton also gets mad at Squidward for making a hideous-looking Krabby Patty.
In the episode "Spy Buddies, Plankton, who is disguised as Mr. Krabs, uses Squidward as cannon ammunition to take down the blimp that is being used by Mr. Krabs, who is disguised as Plankton. This shows that Plankton does not seem to care about Squidward.
In the episode "Krabs à la Mode," Plankton freezes Squidward, although this is a result of him playing with the Krusty Krab thermostat.
In the episode "Bucket Sweet Bucket," after the Chum Bucket is destroyed, Squidward uses its parts to improve the décor of the Krusty Krab. This shows that Squidward cares so little about Plankton that he is willing to use the parts of the Chum Bucket to help the Krusty Krab.
In the episode "Buried in Time" Plankton knocks out Squidward while trying to break into the time capsule.
In the episode "Frozen Face-Off," in the first half, Squidward goes against Plankton, among others to win one million dollars. Later, when Squidward finds out that the race was all a trick by Plankton, he is angry at Plankton, even joining the chase when they get back to Bikini Bottom.
In the episode "Sweet and Sour Squid," Plankton attempts to befriend Squidward to get the secret formula. During most of the episode, Squidward stops every attempt from Plankton to get the formula. Later, Plankton steals Squidward's clarinet and destroys it because he wants to stop Squidward's awful playing of the instrument.
In the episode, "Chum Fricassee," Plankton offers Squidward a position at the Chum Bucket. At first, Squidward is suspicious but takes the job after seeing the alternative, which is continuing to work for Mr. Krabs. At the end of the episode, Plankton and Squidward blame each other for the ruining of Grandma Tentacles' recipe.
In the episode "It Came From Goo Lagoon," when Plankton tries to sell chum from a wheelbarrow, Squidward smashes him with an umbrella. Plankton gets mad at Squidward, then SpongeBob and Patrick land on Squidward with goo balls. Plankton then gets happy and claims it to be karma.
In the episode "Company Picnic," when the illusion of Plankton's company picnic is undone, Squidward is disgusted by the reality of it.
In "Pineapple Invasion" Plankton insults Squidward's nose and had his skunk spray everyone in The Krusty Krab, including Squidward.
In "Salsa Imbecilicus," he made Squidward, along with the whole town dumb as Patrick.
In "Goodbye Krabby Patty?," when Plankton stole the new secret formula the Krusty Krab was selling, Squidward became concerned and told SpongeBob to stop him.
In "Bottle Burglars," Squidward panics after finding out Plankton stole the secret formula, and teams up with SpongeBob to try to steal it back.
In The SpongeBob SquarePants Movie, Squidward figures out that Plankton's plan was to freeze Mr. Krabs, steal the Krabby Formula, and that he was the one who really stole the crown. However, Squidward is too late to stop Plankton as he goes into his next phase, which is to use the Chum Bucket bucket helmets to mind control the Bikini Bottomites. His first order is to make them capture Squidward. Later, Squidward appears as a servant of Plankton until Squidward is freed by SpongeBob.
In The SpongeBob Movie: Sponge Out of Water Squidward goes against SpongeBob and Plankton as they cause an apocalypse in Bikini Bottom.
In the special "Plankton's Special Sinister Commentary," Plankton claims that Squidward creeps him out.