SpongeBob is asleep in his bed and wakes up. He realizes that it is Sunday, so he decides to have an ice cream "sundae" for breakfast. However, he sees he has no normal ingredients (ice cream, hot fudge etc.), so he creates a "sundae" made from onions, ketchup, and peanuts, from a nearly dead plant which 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 rancid 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 (though his eyes get cracked after the scent hits him). He suggests that maybe everybody hates his laugh, and after a long and very irritating chuckle, SpongeBob flatly asks his friend if he is kidding. Patrick then comes to the conclusion that SpongeBob is ugly, and then 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 tells SpongeBob the story of The Ugly Barnacle, thinking it would help, but of course, it does not.
Patrick then attempts to teach him to be proud of his ugliness. It works, and SpongeBob's confidence grows, so they go to The Reef at night.
When they go in and sit down, all of the fish still try to avoid SpongeBob, which angers Patrick and sends him into a maniacal rage. Patrick then forces the other viewers to look at SpongeBob. Disgusted by SpongeBob's breath, the fish panic and flee the theater. Patrick eventually gets hungry, so they go to get a snack. However, because everybody else had fled the theater, no one is at the counter. Instead, SpongeBob pulls some of his sundae out of his pocket and gives it to Patrick. After eating it, Patrick goes to the bathroom.
In the bathroom, a fish runs away from Patrick, which confuses him, as he had only asked for some soap. Patrick then thinks he has caught the "ugly" from SpongeBob, and hides in a stall with a paper bag over his head. Eventually, SpongeBob smells Patrick's extremely foul breath, and comes to the realization that they are not ugly, but simply have bad breath from the sundae. Soon, the two begin to run around shouting "We stink!," melting down The Reef itself with their rancid breath. In the last scene, SpongeBob and Patrick run into Squidward, who is looking at wigs in a window display. SpongeBob says, "Guess what, Squidward? We stink!" Squidward looks confused as SpongeBob and Patrick run away laughing.
SpongeBob refers the Ketchup by its true pronunciation rather than what it says on the product label ("catsup").
When Patrick holds SpongeBob up and shouts "Look at it!" repeatedly has become a popular internet fad called "Patrick Wants You To Look At It," where something else replaces SpongeBob.
The Ugly Barnacle story is one of Patrick's most famous quotes in the entire series.
The shot where Fred is seen screaming has become a major YouTube phenomenon. He has been commonly referred to as the "Deuueaugh fish." Several videos include him screaming that for long periods of time. (Ten hours, ten minutes, etc.). There are also videos of the scream in multiple languages. Currently, the most in a single video is 27.
This is the first episode where Patrick is shown to have a "Z" shape for his eyebrows. With the exception of "Big Pink Loser," all future episodes will show him like this, instead of the classic sideways "M" shape.
However, unlike those previous episodes where SpongeBob is seen wearing glasses, the glasses he wears in this episode are a parody of Groucho Marx.
In the DVD commentary, it is mentioned at the end of this episode that Squidward had a deleted line. After SpongeBob and Patrick hug him and run away, he would say "Ugly and smelly, two for one," but it was cut due to the whole screen fading to black at the end.
For the beginning, the giant calendar that SpongeBob is flung into by his bed is a gag reused from "Boating School."
Running Gag: The police giving a ticket to destroyed cars, tickets meters, etc. or they destroy it.
MY LEG: The marching band gets irritated with the stench of SpongeBob's mouth after they bump into buildings that they got thrown into.
This episode has a pretty high kill count:
Norton's body layers fall apart.
SpongeBob's reflection commits suicide by smashing himself with a hammer.
2 fish commit suicide by reeling themselves up with a hook.
SpongeBob causes a car to explode, killing the driver.
The Ugly Barnacle was so ugly that everyone died.
SpongeBob's bad breath kills 2 fish in the movie theater. Later, when SpongeBob and Patrick run around in joy, their rancid breath eventually melts down the entire theater.
Another fish gets her face destroyed by SpongeBob's breath.
If one looks closely at the title card, SpongeBob's hand can be seen.
There is an online game titled Stinky Swagger, which was based off of this episode.
After SpongeBob finishes his sundae, he says "You know what they say, Gary... I'm easy like Sunday morning," a reference to the lyrics to The Commodores' "Easy."
The shot with SpongeBob playing the organ is a reference to The Phantom of the Opera.
The Ugly Barnacle may be a possible reference to The Ugly Duckling, by Hans Christian Andersen.
When SpongeBob says, "Hello, Dolly!," he references the 1964 musical of the same name, which was later made popular by Louis Armstrong.
A brief glitch can be seen and heard during the title card sequence. This has occurred on Nicktoons and Nickelodeon airings of this episode for several years. There has yet to be an explanation for it.
When SpongeBob talks to the blue woman in the red dress, the fish in the background change colors from blue to green to red.
When Patrick says, "Don't worry about him, SpongeBob," the fish turns back to green.
When Patrick sees SpongeBob in his house playing the organ, SpongeBob has a mask on, but after SpongeBob turns around and Patrick lifts him up, the mask is gone.
When credited as Animation Director, Edgar Larrazábal's last name is misspelled as Larrazabla.
At The Reef, SpongeBob and Patrick first sit in the front row. But later, when Patrick lifts SpongeBob, they appear to be in the middle row.
As Patrick speaks to SpongeBob about how ugly he got, he mentions that he does not have a sister. However, he does, as seen later in "Big Sister Sam."
However, tt could be possible that he forgot about her, due to his stupidity.
At the Reef, when SpongeBob and Patrick get into the theater, it is in a wide-shot and Fred is at the end of the front row. However, when SpongeBob and Patrick pass him, he is at least two seats in and next to Dave.
On the VHS/DVD releases and YTV airings of this episode, the scene where SpongeBob says "hi building" and causes the building to flee from his bad breath plays the following audio: "SpongeBob: I just don't get it! Patrick: I don't either!," instead of what it should be. According to the Season 2 DVD commentary, this was an audio error. It's made obvious since SpongeBob's mouth still clearly says, "hi, building." Strangely enough, the closed captioning still reads, "Hi, building."
The bag on Patrick's head in the VHS, DVD, and YTV versions is brown, instead of blue and yellow striped.
At the Reef, when SpongeBob and Patrick get into the theater, it is in a wide-shot and Tom is next to the red fish, Harold. However, when SpongeBob and Patrick get to Harold, he is sitting next to the red-haired fish.
At the Reef, when SpongeBob and Patrick get into the theater, it is in a wide-shot and almost every seat has someone in it, but when SpongeBob gets to the front row, very few seats are filled.
At several parts in the episode, SpongeBob does not have the smelly breath cloud.
In the bathroom, when SpongeBob first opens the stall door, Patrick's reflection does not move.
When SpongeBob yells "Patrick!," for a split second, a majority of his holes are missing. However, the reflection on the floor shows pores.