- Gary the Snail
- SpongeBob SquarePants
- Johnny Elaine
- Karate artists (debut)
- Sandy Cheeks
- Master Udon (debut)
- Other "karate masters" (debut)
- Karate Island's best fighters (debut)
- The Tickler (debut)
- Lip Service (debut)
- Filthy Phil (debut)
- Squidward Tentacles (cameo)
SpongeBob is waxing Gary when the local mailman shows up at SpongeBob's house and hands him a videotape delivery. SpongeBob plays the tape on his TV, which states that he has been crowned "King of Karate" and tells him to attend to Karate Island to be crowned.
SpongeBob is about to leave when he encounters Sandy, to whom he happily explains that he is the King of Karate. Sandy is suspicious of this, so she accompanies SpongeBob on the boat ride. The boat stops at Karate Island, where the guru, Master Udon, greets SpongeBob but ignores Sandy. SpongeBob, to prove his Karate skills, asks to fight Master Udon's best guards. Three guards arrive, and when they fight SpongeBob, they pretend to get hurt and soon pretend to be defeated.
Sandy notices this, adding to her suspicion. SpongeBob is officially crowned "King of Karate" and sits on his throne and requests Sandy to sit beside him in a lounge chair. Sandy leaves when SpongeBob becomes arrogantly proud and accuses her of being jealous of him. After she leaves, SpongeBob is suddenly locked into his throne. Just as the water is up to Sandy's head (she was walking into the water about to leave), she hears SpongeBob's cries for help, and reluctantly goes back to save him and her usual suit turns into a jumpsuit.
Sandy fights through Master Udon's Tower, fighting an opponent on each floor with an unusual "power" (there are three floors that Sandy has to get through and for all of them, she had to avoid being tickled, kissed and smelling the filth of Udon's minions by using jelly filled donuts, hair dryer and fresh air dome in her helmet) until she crashes through the top floor's entry way; the trapdoor where SpongeBob is being held hostage by Master Udon. Udon reveals that he is a real estate agent, the island is truly Condo Island, and will not let SpongeBob go until he agrees to buy a time-share condo on the island, while also increasing his equity. He then tells SpongeBob he is not the king of karate (as it was just a scam and if there was a real karate island he’d be a millionaire), but could be king of condos. SpongeBob shouts in horror and Sandy attempts to fight Udon, but he drops a sturdy bamboo cage on her, which she quickly breaks out of with meditation, strange hand movements, and weird muttering. Master Udon rips off his robes to expose his speedo while Sandy rips off her jumpsuit to expose her bikini and after a brief muscle growth in their swimsuits (Udon's extra muscles were in his eyebrows, and Sandy's were in her tail) they begin fighting by charging at each other but they miss and Sandy defeats Udon by kicking him out of the room into the water below and she frees SpongeBob. As they leave the island, SpongeBob apologizes to Sandy for acting like a jerk to her and thanks her for saving him from buying a condo, and which she forgives him and tells him that's what friends are for. Then SpongeBob asked if he's not the king of karate anymore, Sandy tells he is in her book. Then she explains to him about real estate as the boat leaves shore.
The episode ends with Squidward arriving in "Clarinet Island" to be crowned "King of Clarinets," and him being looked down upon by Master Udon.
The episode originally ended with a dedication to Noriyuki Pat Morita (1932–2005), best known as Arnold from the TV show Happy Days and Mr. Miyagi from The Karate Kid series, who voiced Master Udon in this episode and died on November 24, 2005. The dedication card at the end of the episode was removed after the Best Day Ever marathon. However, the online version, and the Polish, Canadian, Australian, Belgian, French, Turkish, Spanish, German, Philippine, the British Nicktoons airings, and DVD versions still have the dedication.
|( ‣ ) Associated production music|
|( • ) Original music|
|( ◦ ) SpongeBob music|
‣ Earl's Revenge - Sage Guyton, Jeremy Wakefield [Gary's shell waxing]
‣ Romeo & Juliet Overture - Peter Ilyich Tchaikovsky, Fiachra Trench ["Special delivery, just for me?"]
‣ Vibe Q Sting - Nicolas Carr ["A videotape?"]
‣ TV Logo [#11.01] - Terry Day [SpongeBob puts tape in VCR]
‣ Target - Hans Ehrlinger [Karate Island video]
‣ Marlin March - Sage Guyton, Jeremy Wakefield ["Did you hear that, Gary?"]
‣ Goofy Conversation - Nicolas Carr, Barry Anthony ["There's something rotten in the Alamo."]
‣ The Jovial Buccaneers [#33.02] - John Fox [SpongeBob and Sandy in a boat]
‣ Unknown Track 46 - Nicolas Carr [they arrive at Karate Island]
‣ Steel Sting - Jeremy Wakefield ["Oh, brother."]
‣ Unknown Track 46 - Nicolas Carr ["Please show me some of your moves, Master SpongeBob."]
‣ Eastern Journey - Jack Beaver, Ronald Hanmer [Udon's best fighters]
‣ Take That! (a) - Helene C.E. Muddiman [SpongeBob "defeats" them]
‣ Lap Steel - Nicolas Carr ["Something smells like rancid rodeo around here."]
‣ Unknown Track 46 - Nicolas Carr ["Excellent."]
‣ Vivat Regina - Laurie Johnson ["We must make ready for your coronation."]
‣ Unknown Track 42 - choir
‣ Parting of the Clouds - Nicolas Carr ["My throne..."]
‣ Classy Harpsichord - Nicolas Carr ["Come, Sandy."]
‣ Unknown Track 46 - Nicolas Carr ["Master, don't worry about her."]
‣ Reinstating the King - Laurie Johnson ["Let the coronation begin!"]
‣ Unknown Track 47 - SpongeBob chained to his throne
‣ Crescendo Fin 10 C - Marcin Pospieszalski [Master Udon reaches for his pen]
‣ Hawaiian Link (b) - Richard Myhill [Sandy leaving]
‣ Towerstreet 17 [#16] - Gerhard Narholz ["SpongeBob's in trouble!"]
‣ Closing the Door - Nicolas Carr ["She will never reach you. She must first pass through the 'Four Floors of Fear'."]
‣ Crime Squad - John Cameron [The Tickler]
‣ Towerstreet 17 [#15] - Gerhard Narholz [Lip Service]
‣ Towerstreet 17 [#4] - Gerhard Narholz [Filthy Phil]
‣ Crime Squad - John Cameron [Sandy bursts through the floor]
‣ Unknown Track 68 - "Release SpongeBob!"
‣ Unknown Track 46 - Nicolas Carr ["He is forbidden to leave until he signs this contract."]
‣ Steel Licks (a) - Jeremy Wakefield ["You mean this whole thing was a scam to get us to buy real estate?"]
‣ Steel Licks - Jeremy Wakefield ["If there was a real Karate Island, I'd be a millionaire."]
‣ Spring Cleaning [#5] - Harry Bluestone, Emil Cadkin [Udon explains]
‣ Dramatic Impact 2 - Ivor Slaney ["Don't do it, Sandy!"]
‣ ? [Sandy chanting]
‣ The Two Ronnies - Alan Stanley Tew ["I won't give in to your timeshare vacation scam!"]
‣ Vibe Sting - Nicolas Carr [Sandy and Udon run past each other]
‣ The Two Ronnies - Alan Stanlew Tew [Sandy and Udon fight]
‣ Alekoki - Kapono Beamer [SpongeBob and Sandy leave the island]
‣ Unknown Track 46 - Nicolas Carr [ending]
- This episode is available on the Karate Island, Complete Fourth Season, Season 4 Volume 2, and First 100 Episodes DVDs.
- "Karate Island" was ranked #1 during the Best Day Ever event on November 9–10, 2006.
- As of June 10, 2019, The Internet Movie Database (IMDB) gives this episode a rating of 7.5/10 based on 358 ratings by users. TV.com users give this episode a rating of 6.7/10 based on 187 votes.
- In SpongeBob's Atlantis SquarePantis during the commentary of Squidward Super Shooter, SpongeBob mentions Sandy fighting against Master Udon.
- This episode is dedicated to Pat Morita (voice of Master Udon), who died on November 24, 2005, due to a kidney failure.
- According to The Ultimate SpongeBob SpongeBash, Jennette McCurdy rated this episode as her favorite.
- In Germany, this episode is called "Der König des Karate," which translates to "The King of Karate."
- In the Greek version of this episode, Squidward says "Είμαι εδώ, είμαι ο βασιλιάς του Καράτε και είμαι εδώ," which means "I am here, i'm the king of Karate and i'm here," but he should say "Clarinet" instead of "Karate."
- The Tickler's mouth full of jelly-filled donuts that Sandy uses to attack is similar to Patrick's mouth full of snowballs that SpongeBob uses in "Snowball Effect."
- In the scene where Sandy fights the enemies on each floor, the dialogue is dubbed, similar to old-style Kung-Fu films.
- For years, there were rumors that in the original airing of this episode, there was some blood on Master Udon's face after Sandy beat him up. Vincent Waller eventually debunked this. 
- Sandy's air helmet protects her from Filthy Phil's body odor, but in other episodes, it doesn't protect her from any smell at all, such as Squidward's belch stench in "Squidtastic Voyage."
- This is the first episode to use sound effects on a title card.
- Ironically, none of The Four Floors of Fear use karate except when Sandy fights Master Udon.
- This episode is the first episode to have an animated title card with sound effects added as well as the title card music.
- This episode marks the first time that "Clownfish Capers" plays.
- In this episode, SpongeBob was afraid of signing a contract for a condo and becoming rich, but in "Chocolate with Nuts" he tries to become rich along with Patrick.
- The painting of a boat inside Gary's shell is a replica of the painting over the Simpson family's couch.
- Udon is named after a Japanese/Korean noodle soup. When Sandy defeats him, she says, "You're in the soup now, Udon!"
- There may be some references to Naruto. As when breaking out of the cage, Sandy does hand seals and she breaks out.
- This episode contains many references to the films Game of Death and Kill Bill: Sandy's yellow jumpsuit is a reference to the jumpsuit that both Bruce Lee and Uma Thurman wore in their respective films. Also, the concept of fighting an opponent on each floor is also present in both of these films.
- This episode is also similar to a series of episodes in the Red Ribbon Army saga of Dragon Ball, where Goku tackles six floors of challenges.
- Sandy's and Udon's muscle growth is loosely based on Toguro's 100% muscle growth in the anime Yu Yu Hakusho.
- When Sandy kicks Master Udon in the face, she is shown with five toes, but she usually only has three.
- When Master Udon throws Sandy in the wall, her helmet does not break.
- SpongeBob's teeth are unusually large at the beginning.
- SpongeBob manages to get out of the chair that he was cuffed to.
- The chair that SpongeBob was cuffed to manages to get to the top floor of the Four Floors of Fear.
- Sandy's costume changes after the scene SpongeBob screams "I can't handle this myself."
- Whenever The Tickler talks, his voice is out of sync.
- However, this may be a parody of poorly dubbed anime.
- When Sandy throws jelly-filled donuts at The Tickler, the first shot of the plate shows that there are barely any left, but the second shot shows the plate full of donuts again.
- Sandy said she taught SpongeBob all about fighting, but in "Tea at the Treedome," he knew karate before they met.
- Sandy goes up to the third floor but slips and falls on her way because it is all covered in mud.