Things are going well at the Krusty Krab, until suddenly an alarm blares, signaling an attempt by Plankton to steal a Krabby Patty. Plankton, controlling the patty, escapes outside with it by making a hole in the wall with a laser. SpongeBob heroically chases after it. After a long chase, SpongeBob finally recovers the Krabby Patty, and is congratulated by the townsfolk, while Plankton walks away sadly. SpongeBob, feeling sorry for Plankton, decides to befriend him. SpongeBob invites Plankton to jellyfish with him, and, realizing he can take advantage of SpongeBob, Plankton goes. Once there, Plankton remains fixated on evil and destruction, and SpongeBob tries to explain fun by singing The F.U.N. song.
Eventually, Plankton starts singing the song as well, and seems to finally be having fun. He seems to forget about stealing the formula, and begins to develop a genuine sense of friendship. When Mr. Krabs sees this, he refuses to believe that Plankton has changed and sets up a trap for them when they come to the Krusty Krab together the next day. He attempts to tempt Plankton with a Krabby Patty, but he seems not to take the bait.
After SpongeBob and Plankton leave to go to the movies, Mr. Krabs begins to believe that Plankton really has gone straight, until he realizes that he actually did take the patty. At the cinema together, Mr. Krabs appears in front of the projector and shouts "reach into his pocket now and take what he's got!". SpongeBob reaches into Plankton's pocket and finds a Krabby Patty (the other people simply steal from their neighbor's pocket and leave the theater). SpongeBob realizes that Plankton used him to get a Krabby Patty, and Plankton attempts to explain that he realizes now that he only ever wanted friendship, before admitting to everything and running off with the patty. However, he runs directly through the movie screen into a solid wall.
Plankton is squashed, and Mr. Krabs flicks him off the wall onto Bubble Bass' hand, who mistakes Plankton for a jellybean and chases him around his hand.
The movie that SpongeBob and Plankton are watching changes throughout the scene. At first it's a fishing movie, which is ironic, and then changes to a romance movie, which allows an ironic joke after the actor says: "Nothing can ever tear us apart" as Plankton destroys the screen.
Plankton hits the wall on the very bottom, but is several inches above the ground when Mr. Krabs reveals the wall.
In real life, the speakers are behind theater screens.
When Plankton stole the Krabby Patty, it doesn't show him stealing it.
This is the first episode to have a designable text in the title cards and have a black background.
The Chum Bucket's special of the day was Chumbalaya, a parody of Jambalaya.
"F.U.N." is the only episode name to be in acronym form.
When Squidward says, "How does it feel to be the most hated thing in Bikini Bottom, Plankton?", he ends it with, "It hurts, doesn't it? I know!", possibly suggesting that Squidward knows he too is disliked by the people of Bikini Bottom.
The F.U.N. song is sung during the SpongeBob Krabby Patty Celebration at Nickelodeon Suites in Orlando.
If you listen carefully, the boot squeak sound effect from Squeaky Boots can be heard when the policeman puts the "Honorary Rookie" donut on SpongeBob.
The adventure music heard when SpongeBob chases after the patty is a parody of the Indiana Jones theme music.
The Krabby Patty appears bigger than usual.
When Plankton is talking to Karen, and he asks "What are you talking about?", the captions say "What do you mean?".
Captions misspell "Bubble Bass" as "Bubble Bath". This also happened in Pickles.
The captions also misspell "SpongeBob" as "Sponge Bath", as seen when Karen tells Plankton to befriend SpongeBob so he can steal the Krabby Patty.
When Karen says: "What did you do with the Krabby Patty?", captions say: "Where's the Krabby Patty?". When she says to Plankton: "You're forgetting the mission", captions say: "Remember the mission?".
Plankton is slightly taller than normal.
When SpongeBob is thrown in the air, the sky is green. But when he lands, the sky is blue.
When the movie theater is first shown, there are eighteen people sitting inside, but in later scenes, there are less.
When SpongeBob is looking for the patty with binoculars, the fishfolk are walking slowly in lines. But when he spots the patty, they begin walking faster.
When Plankton is leaning against the door of the Chum Bucket, he appears to be as big as the door.
When SpongeBob chases the patty off a building, he uses his blown-up hat to bounce about town. But when he catches the patty at the magic shop, his hat was back on his head and deflated.