SpongeBob SquarePants is asleep in his bed then wakes up and realizes that it is Sunday, and therefore decides to have an ice cream sundae for breakfast. However, he sees he has no normal ingredients (Ice cream, etc.), so he creates a "sundae" made from onions, ketchup, and peanuts from a peanut plant he had been growing in his window. The sundae gives SpongeBob halitosis, which he does not realize. After breakfast, SpongeBob begins his Sunday to-do list, which consists of saying "hi" to everyone in town.
Everyone in town is disgusted by the smell of SpongeBob's breath, and avoids him. He is very confused by this, and asks Patrick if he notices anything wrong with him. Patrick, not having a nose, is unaffected by his foul breath. He suggests that maybe everybody hates his laugh, and after a long and very irritating chuckle; SpongeBob flatly asks his friend if he's kidding. Patrick then comes to the conclusion that SpongeBob is ugly, and does a "Reflection Test". SpongeBob is shocked by this, and isolates himself in his home.
Later, Patrick visits SpongeBob, who has now become a hermit. Patrick attempts to teach him to be proud of his ugliness.
It works, and SpongeBob's confidence grows, so they go to the cinema. When they go in and sit down, all the fish still avoid SpongeBob. Angered that no one has the nerve to face SpongeBob's so-called ugliness, Patrick goes into a maniacal rage and forces the other viewers to look at SpongeBob. They all panic and run out of the theater. Patrick eventually gets hungry, so they go to get a snack, but no one is at the counter, so SpongeBob gives Patrick some of his sundae. Then Patrick goes to the bathroom. When a guy runs away from Patrick in the bathroom, he thinks he has caught the "ugly" from SpongeBob. SpongeBob smells Patrick's extremely foul breath, and comes to the realization that they are not ugly, and simply have bad breath from the sundae. After they realize this, the two begin to run around shouting "WE STINK!", melting down the movie theater with their corrosive stench. In the last scene, SpongeBob and Patrick run into Squidward, who is looking at wigs in a window display, and SpongeBob says, "Guess what, Squidward? WE STINK! dahahaaaahahahaaahahhaahaahahaahha!" Squidward looks confused and SpongeBob and Patrick run away laughing.
The Ugly Barnacle story in this episode has spawned extreme popularity; it has been subject to remixes and edits.
The scene where Patrick takes the paper bag off his head and tells SpongeBob he "caught" the "ugly" has also become a meme on Youtube.
The scene where Fred goes "DEUUEAUGH" has also become a popular hit on YouTube.
When SpongeBob is playing the organ, his sleeves are pink instead of white. This is because of the firelight. Also, when he is first shown playing the organ, he wears a mask, but after the camera shows Patrick (and then it shows SpongeBob) the mask disappears. Although he could have taken it off offscreen.
SpongeBob refers the ketchup by its true pronunciation rather than what it says on the product label "katsup."
This is the first episode to use traditonal cel animation instead of digital ink and paint.
After SpongeBob finishes his sundae, he says You know what they say Gary...I'm easy like Sunday morning., a reference to the song "Easy Like Sunday Morning".
SpongeBob says "Hello, dolly!" in the marching band scence, a reference to the film "Hello, Dolly!"
In the original TV version, SpongeBob walks up to a building and says "Hi, building!" which causes the building to flee from SpongeBob's breath. He goes up to Patrick and says "I just don't get it!" and Patrick says "I don't either." In the Season 2 DVD release, as well as the YTV version, the scene goes like this:
SpongeBob: (walks up to a building and waves) I just don't get it! (the building flees)
Patrick: I don't either. (SpongeBob walks up to Patrick; the rest of the scene goes as normal, resulting in the same lines being repeated)
In the DVD commentary for this episode, they mention that Squidward had a deleted line at the end of the episode. After SpongeBob and Patrick hug him and run away, he would say "Ugly and smelly, two for one."
While SpongeBob shows the Peanut onion sundae to Patrick the smell burns Patrick's eye brows but in the next scene the eye brows were back.
It is confirmed in this episode that Patrick doesn't have a nose, but in "Wet Painters", Patrick has a nose (or a hole, as Patrick only has one nose opening). Also, in "Suds", Patrick puts a clothespin over where his nose would be.
At the movies, SpongeBob and Patrick at first sit in the front row, but later, when Patrick lifts SpongeBob, they appear to be in the middle row, not the front.
This is the first episode where Patrick is shown to have "Z"s for eyebrows, the second being "Sandy,SpongeBob, and the worm".
Patrick is shown washing his hands, but in "The Battle of Bikini Bottom", he claims to never have washed them (although he does not actually get to washing them in this episode).
MY LEG: SpongeBob uses a big ball of breath to knock a marching band.
This is the only time a didgeridoo is seen on the show.
When SpongeBob says hi to a girl at the movie, she suddenly disappears in the next scene.
In this episode, Patrick said he didn’t have a sister. But later he does in Big Sister Sam. Though it is most likely that Patrick forgot about his sister, considering his low intelligence and that she was lost long ago.
SpongeBob says "Hi, mailfish" even though mail doesn't come on Sunday.
SpongeBob points out a giant pile of pink bubblegum (which turns out to be Patrick), but interestingly, in The Gift of Gum, he would receive a giant pile of pink bubblegum as a gift, from Patrick.
After Fred yells My Leg, one of the marching band members yells out "Oh my God!"
When SpongeBob says hello to each member of the marching band, he greets the timbale man. Timbales are shallow, single-headed drums invented in Cuba.
The sea onion is a real plant, but it's an ornamental house plant rather than an edible vegetable.