Squidward is still asleep, only to be woken up and then annoyed by SpongeBob and Patrick, who came into his house to throw a surprise birthday party for him. When they leave, he shouts that it isn't his birthday and calls Patty Rechid from "Bikini Bottom Realty" to arrange for his house to be sold, so that he may move out of Bikini Bottom. He is warned that if his home is surrounded by bad neighbors, the house may not be sold. Squidward is convinced that SpongeBob and Patrick may ruin his chances of selling his house. Squidward thinks that whatever a good neighbor does, SpongeBob and Patrick do the opposite, which gives him an idea.
Squidward tells SpongeBob that it is "Opposite Day" in the next morning, and that everyone must act opposite to how they usually act. SpongeBob believes him, and informs Patrick, who stops breathing, until SpongeBob exclaims, "not that opposite!" A montage of the two doing "opposite" things, such as talking backwards and acting as Gary's pets, is then shown.
Squidward's plot soon begins to backfire on him. SpongeBob "fixes" (damages) his house, and a shocked Squidward cleans the debris with a bulldozer. SpongeBob concludes that Squidward is acting like him, and he must therefore act like Squidward. While Squidward is still fixing SpongeBob' house, Patty arrives, and SpongeBob and Patrick both pretend to be like Squidward, giving her a tour of the house, while describing it negatively and doing the opposite of what she asks. When Squidward realizes what is going on, he rushes to his house and attempts to explain the situation to Patty, who, upon hearing that he is "Squidward" as well, finally sees through his game and accuses them of intentionally messing with her. Gary comes in with a pickle on his face, resembling Squidward's large nose. He meows with a Squidward-style voice, and she leaves. Squidward begs Patty to sell his house, only for her to yell at Squidward by saying "NEVER!" and she wouldn't sell the house if he was the "last Squidward on Earth" and then she storms off. SpongeBob and Patrick tells Squidward "Happy Opposite Day." A furious Squidward shows SpongeBob and Patrick how much he "hates" them by attempting to run them over with a bulldozer, scaring SpongeBob and Patrick out of their wits as he is now on the warpath against them.
Despite it being Tuesday and Wednesday, SpongeBob and Squidward were not working at the Krusty Krab, but since SpongeBob actually thought it was opposite day he didn't go to work or they had a few days off.
It is revealed in this episode that SpongeBob knows that Squidward dislikes him.
In the Russian dub, the line "Kcirtap yeh. Pu evig I. Edis etisoppo eht ot teg ot." was changed to "Кем мы были? Пузырями. Сами надулись и сами лопнули!" ("Who were we? Bubbles. We blew ourselves & popped ourselves!"). The line isn't spoken backwards.
In Indonesia, instead speaking backwards, SpongeBob and Patrick are saying the riddle of "Why did the chicken cross the road?"
This episode takes place after "Sing a Song of Patrick" because the framing adds to SpongeBob's house in this episode
This episode takes place after "Rodeo Daze" because SpongeBob's pineapple was a real fruit in that episode, but not in this episode.
This is the first time Gary has said something other than "Meow". He barks like a dog instead of his usual meowing.
SpongeBob impersonates Squidward for a second time, the first time being in "Ripped Pants".
During the "Pin the Tail on the Seahorse" scene, the tail that Squidward is holding is not a seahorse's tail. Also, the image of the seahorse already has a tail.
When Squidward's house is seen by itself after Squidward said "Opposite" after having known how to solve the issue of SpongeBob and Patrick's success of preventing Squidward selling his house, SpongeBob and Patrick's house don't appear.
When Squidward is rebuilding SpongeBob's house, he just started. When the view changes to SpongeBob for one second, the modelling is already finished. Or maybe Bikini Bottom time is faster then real time.
When Squidward starts to rebuild SpongeBob's house, the lower part of his body is not colored in (which is hard to tell at first since the sky was already blue).
When SpongeBob puts the belt around his head, the belt buckle and the piece of belt that goes through it disappears.
When Patrick first sings "I'm Squidward, I'm Squidward, Squidward, Squidward, Squidward" the flowers on his pants are almost black, but in the next shot they are dark purple - like in other episodes in the first season.
When Patty walks into Squidward's house, Patrick's house cannot be seen.
When Patty says, "Are there anymore Squidwards I should know about?," the closed-captions said, "Are there anymore Squidwards?"
After Patty leaves, Squidward lays on the ground in shame. In the close up of his face, SpongeBob has his belt around his waist, but it was wrapped around his head.
It's possible that Spongebob took off his belt and put it back on his waist off screen
In one scene, an ear on Squidward's house has a window, then it's gone.