Squidward Tentacles, 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."
While it is a minor occurrence, the next time they were shown working together was in "Sleepy Time," where the two are among a group of people, who are angry that Dream SpongeBob invaded their dreams.
While it is unknown if Squidward was in on the ruse, in the episode, "Spy Buddies," Squidward and Mr. Krabs, who is later revealed to be Plankton in disguise, have a few scenes together, one of which shows Squidward points out that Plankton, who is actually Mr. Krabs in disguise, is nearing The Krusty Krab.
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.
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, "Eek, an Urchin!," Plankton helps Squidward, Mr. Krabs, and SpongeBob to get rid of sea urchins, which have invaded The Krusty Krab.
In the episode, "Company Picnic," Plankton convinces Squidward and Mr. Krabs to join his company picnic.
In the short, "Leo," Squidward uses Plankton in a lion outfit as Leo.
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 resturant.
The next occurance is in "Plankton's Army," where Plankton, along with his family members, stuffs Squidward and Mr. Krabs into a toilet in Mr. Krabs' office in The Krusty Krab.
The next occurance, while it is more one-sided, is in "Fear of a Krabby Patty," where Plankton adds a sign to the Chum Bucket that advertises the resturant is opened 23 hours a day. This makes Mr. Krabs change his sign to being open 24 hours, which causes Squidward to be overworked.
In the episode, "Spy Buddies," Plankton, who is disguised as Mr. Krabs, uses Squidward as canon amunition 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's well-being.
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 decor 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. Although, it may have just been a case of revenge since earlier in the episode, Squidward paints the Chum Bucket and Plankton does not like, even painting over Squidward's painting.
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 Bikni Bottom.
In the episode, Sweet and Sour Squid during most of the episode, Squidward stops every attempt from Plankton to get the formula. Later, Plankton destroys Squidward's clarinet 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 blames each other for the ruining of Grandma Tentacles' recipe.
In the episode, "Company Picnic," when the illusion of Plankton's company picnic is undone, Squidward is disgusted by the reality of it.
In "Salsa Imbecilicus," he made Squidward, along with the whole town dumb as Patrick.
In The SpongeBob SquarePants Movie, Squidward figures out that Plankton's plan to freeze Mr. Krabs and that he 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 caused an apocalypse in Bikini Bottom.
In the special "Plankton's Special Sinister Commentary," Plankton claims that Squidward creeps him out.