SpongeBob wakes up, and realizes that it's April Fools Day. He then begins the day pranking Gary by saying they were going to be peasants and that they were going to have to move. He then pranks himself by saying that he was going to drink several glasses of lemonade, then goes to work. Squidward sees him and wonders what SpongeBob is happy about until he looks at the calendar, and realizes that it's April Fools Day, which he says is SpongeBob's favorite holiday. Fearing the worst has to come (or so he thinks), Squidward calls Mr. Krabs saying he's sick and that he would not be able to make it to work. Mr. Krabs says that there's an art collector in his office who wants to buy Squidward's art and name him artist of the month. Squidward immediately rushes to the Krusty Krab, only to find that it was a silly prank by SpongeBob and Mr. Krabs. Squidward attempts to resign from his job, but SpongeBob convinces him to stay, promising not to pull any further pranks on him.
However, SpongeBob continues to prank the customers inside the Krusty Krab while hiding them from Squidward even though Squidward knows he is doing that. The customers seem to take SpongeBob's pranks lightly, because the pranks are considered funny and harmless. Despite the fact that Squidward does not receive any more pranks as SpongeBob promised, Squidward gets annoyed over the SpongeBob and the customers' laughter. He decides to plot revenge on SpongeBob by showing him what a "real" prank is, and sets a rope trap with a blob of mustard in the middle. SpongeBob goes to clean it up, and he gets his leg caught in the lasso. Squidward cuts a nearby rope, triggering a chain reaction which sends SpongeBob flying around the restaurant, colliding into several objects and areas, ending with him into landing in a garbage can. Squidward laughs hysterically at him and celebrates his victory, but an emotionally and physically hurt SpongeBob runs out of the restaurant in tears. Bewildered by Spongebob's reaction, Squidward calls out to him, saying that he was just kidding, even turning to the customers for confirmation. But the customers too are appalled by Squidward's prank, declaring that it was far too harsh to be considered funny. They all leave the restaurant in disgust, with one of them angrily saying to him "April Fools, jerk." and another one yelling, "YOU STINK!" Squidward insists that it was only a joke, but as he looks around at the damage to the restaurant that his prank caused, and conversely realizes how Spongbob was hurt during each part, it slowly dawns on him that Spongebobs pranks we're actually funny and meant to make people laugh, while his prank was cruel and meant to hurt and humiliate Spongebob. Overcome with guilt, and realizing that they were right in that the prank was too harsh, he decides that he has to apologize to SpongeBob.
He goes to SpongeBob's house and sees SpongeBob's hat behind a rock. Thinking it's SpongeBob, he goes over to apologize, but then sees that it's Patrick wearing the hat and digging. Squidward asks where SpongeBob is and Patrick tells Squidward that SpongeBob is in the house "impressed" (instead of depressed) and was crying a lot. Squidward feels even worse and attempts to apologize when SpongeBob answers the door, but finds himself physically unable to say "I'm sorry" to him. Eventually, he is able to say it by putting a bubble over his head, so that SpongeBob does not actually hear his apology. Squidward walks away, saying that his conscience is clear, but is confronted by memories of what he did, feels guilty again, attempts to apologize to SpongeBob several unsuccessful times, and eventually goes back and genuinely apologizes to SpongeBob, but tells SpongeBob not to tell anyone. SpongeBob promises not to tell and forgives him, but then fully opens his front door, revealing that everyone else is inside behind him, and witnessed Squidward's apology. They all say "April Fools!" to Squidward, who is flabbergasted by this. He states that he was fooling them as well before running away to his house while laughing maniacally.
Even though Squidward said April Fools Day was SpongeBob's favorite holiday, it is mentioned by SpongeBob that his favorite holiday is Christmas at the start of It's a SpongeBob Christmas!, leading to confusion.
This episode takes place before Christmas Who?, which is why April Fool's Day is SpongeBob's favorite holiday, as he and the other Bikini Bottomites (With the exception of Sandy Cheeks) don't know that Christmas exists.
Directing an old lady who asked him for a fork to the spoons
Drawing a face on the back of his head and tricking a customer into thinking he was two different people
Giving a customer one ice cube in his drink when he asked for a couple of them
Telling his tongue that its shoes were untied
Tricking Squidward into apologizing to him secretly in front of a room full people, including those who witnessed the prank at the Krusty Krab
The line "You stink!" was first heard in "Pickles".
Squidward's Mom looks different than she does in "Krusty Towers."
The woman who asks SpongeBob where the forks are is the same fish that asks where the menu section is in "Neptune's Spatula." It seems as though she is in constant need of guidance..
When Squidward tries to apologize to SpongeBob for the first time, he says "I'm trying to say I'm..." and his head turns into a donkey and starts braying. A similar subliminal message will also occur in "Christmas Who?".
This episode and "Neptune's Spatula" only tend to air on April Fools Day, as Neptune's Spatula is April Fool's Day's sister episode. However, recently, airtime of these episodes became a bit more common.
After being pranked the first time, Squidward places his hat on Mr. Krabs's desk, but he has it in his hand when he leaves the Mr. Krabs's room.
Even after SpongeBob reveals to the old lady his prank of giving her a spoon rather than a fork, she still walks away with the spoon.
SpongeBob shouldn't be able to talk properly when his tongue fell off.
When the customers leave the Krusty Krab after Squidward's prank, they are all blue and brown finger shaped ovals.
After SpongeBob says "What" while Squidward is trying to apologize, the doorknob reappears eventhough SpongeBob took it down just 15 seconds earlier
When Squidward's head explodes while apologizing, neither his house nor Patrick's house are there anymore
After Squidward leaves SpongeBob's house, an image of Tom says "April Fools, jerk!". However, a different fish says this at the Krusty Krab. That fish was lime green with dark details.
When Squidward gives SpongeBob a message in a bottle, he puts a cork on it. However, as it rolls towards SpongeBob, the cork is gone.
When Squidward sees Patrick, the hat is down to Patrick's eyebrows. However, when Squidward says, "Why are you wearing that hat?!" the hat is farther up Patrick's forehead.
In one scene SpongeBob's holes were black.
Squidward finishes saying sorry before he stuck his tongue out.
In this episode, Bubble Bass is purple (or brown), instead of green.
There is no possible way that many people can fit in SpongeBob's living room, and when Squidward was apologizing, he said that he "liked everyone" which means that maybe the whole of Bikini Bottom was in SpongeBob's living room. Unless it was the people just at the Krusty Krab and SpongeBob's friends, then they would fit into it.
Shubie is heard speaking in a deep male voice.
When SpongeBob and the others says "April Fools!" to Squidward, SpongeBob's mouth did not move.