- Gary the Snail
- SpongeBob SquarePants
- Incidental 154
- Realistic Fish Head
- Karate artists (single appearance)
- Sandy Cheeks
- Master Udon (debut)
- Other "karate masters" (single appearance)
- Karate Island's best fighters (debut)
- The Tickler (debut)
- Lip Sylvest (debut)
- Filthy Phil (debut)
- Squidward Tentacles (cameo)
- Pat Morita (special thanks appearance at the end)
In the beginning of the episode, SpongeBob is waxing Gary, when the local mailman shows up at SpongeBob's house, and he 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 he sits on his throne and requests Sandy to sit beside him in a lounge chair. Sandy tries to warn SpongeBob, but then leaves when SpongeBob becomes arrogantly proud and accuses her of being jealous of him. After she leaves, SpongeBob is suddenly locked into his throne. She then hears SpongeBob's cries for help, and 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 the King of Karate thing is just a scam, and he is actually a real estate agent, the island is truly Condo Island, there is no such thing as Karate Island (if there was, he would be a millionaire), and he will not let SpongeBob go until he agrees to buy a time-share condo on the island, while also increasing his equity. 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 he thanks her for saving him from buying a condo, for which she forgives him and tells him that is what friends are for. SpongeBob then asks if he is not the king of karate after all, but Sandy tells him that he is in her book. Then she explains to him about real estate as the boat leaves shore. As Squidward arrives on "Clarinet Island" to be crowned "King of Clarinets," he then looks down upon by Master Udon.
- Sandy goes up to the third floor, but slips and falls on her way because it is all covered in mud.
The episode originally ended with a dedication to Noriyuki "Pat" Morita (1932–2005), who voiced Master Udon in this episode and died on November 24, 2005 due to kidney failure. He was best known as Mitsumo "Arnold" Takahashi from the TV show Happy Days, Mr. Miyagi in The Karate Kid, and the Emperor of China in the Walt Disney Animation Studios 36th animated feature film, Mulan. The dedication card at the end of this episode was removed in American television airings after the Best Day Ever marathon. It has also been removed from Paramount+. However, the Polish, Canadian, Australian, Belgian, French, Turkish, Spanish, German, Greek, Russian, Asian, Philippines, the British Nicktoons airings, and DVD versions still have the dedication intact.
|( ‣ ) Associated production music|
|( • ) Original music|
|( ◦ ) SpongeBob music|
• Clownfish Capers - Sage Guyton, Jeremy Wakefield [Title card]
• 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 2 - Nicolas Carr, Barry Anthony ["There's something rotten in the Alamo."]
‣ The Jovial Buccaneers [#33.02] - John Fox [SpongeBob and Sandy in a boat]
• Welcome to Karate Island - Nicolas Carr [they arrive at Karate Island]
• Steel Sting - Jeremy Wakefield ["Oh, brother."]
• Welcome to Karate Island - Nicolas Carr ["Please show me some of your moves, Master SpongeBob."]
• Karate Gong - Nicolas Carr [Fighters appear]
‣ 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."]
• Welcome to Karate Island - Nicolas Carr ["Excellent."]
‣ Vivat Regina - Laurie Johnson ["We must make ready for your coronation."]
• Revelation - Nicolas Carr [choir]
• Parting of the Clouds - Nicolas Carr ["My throne..."]
• It's Good to be King - Nicolas Carr ["Come, Sandy."]
• Welcome to Karate Island - Nicolas Carr ["Master, don't worry about her."]
‣ Reinstating the King - Laurie Johnson ["Let the coronation begin!"]
• Entrapped 2 w/ Closing the Door Sting - Nicolas Carr [SpongeBob chained to his throne]
‣ Hawaiian Link (B) - Richard Myhill [Sandy leaving]
‣ Towerstreet 17 [#16] - Gerhard Narholz ["SpongeBob's in trouble!"]
• The 3 Challenges - 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]
• Tension Threat - Nicolas Carr ["Release SpongeBob!"]
• Welcome to Karate Island - Nicolas Carr ["He is forbidden to leave until he signs this contract."]
• Steel Licks 8 - Jeremy Wakefield ["You mean this whole thing was a scam to get us to buy real estate?"]
• Steel Licks 9 - 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!"]
• ? - Nicolas Carr [Sandy chanting]
‣ The Two Ronnies - Alan Stanley Tew ["I won't give in to your timeshare vacation scam!"]
• What's This - Nicolas Carr [Sandy and Udon run past each other]
‣ The Two Ronnies - Alan Stanlew Tew [Sandy and Udon fight]
• Karate Gong - Nicolas Carr [Udon falls]
‣ Alekoki - Kapono Beamer [SpongeBob and Sandy leave the island]
• To Pat Morrita From NC - Nicolas Carr [ending]
- This episode is available on the Karate Island, Season 4 Volume 2, The Ultimate Box Set, Sportingly Sandy, First 100 Episodes, Best of SpongeBob, Double Feature Volume 1, Double DVD Fun, The Ultimate SpongeBob Box Set, SpongeBob SquarePants Vol. 6, Rockin' Bikini Bottom, Complete Fourth Season, Extreme Kah-Rah-Tay (digital version only), From the Beginning, Part 2, The SpongeBob SquarePants Collection, The SpongeBob SquarePants 8 Season DVD Collection, Favorite Episodes Volume 2, 2-Season TV Pack, The Third & Fourth Seasons, and The Best 200 Episodes Ever DVDs.
- "Karate Island" was ranked #1 during the Best Day Ever event from November 9–10, 2006.
- "Karate Island" was ranked #73 during the SpongeBob's Top 100 event in the UK and Ireland from June 4-8, 2012.
- This episode has an animated title card.
- This is the first episode to use sound effects on a title card along with the title card music. The sound effect that played was karate chops.
- This is also the second episode to have the title card shown in the actual episode. The first is "Krabby Land."
- This episode marks the first time that "Clownfish Capers" plays.
- The yellow jumpsuit that Sandy was wearing later appears in the video game The Cosmic Shake and the crossover app game Super Brawl Universe.
- In the scenes where Sandy fights the enemies on each floor of the Four Floors of Fear building, the dialogue is dubbed, similar to old-style kung-fu films.
- Innuendo: One of the enemies Sandy fights is a "French Tickler."
- Sandy's air helmet protects her from Filthy Phil's body odor, but in other episodes, it does not protect her from any smell at all.
- 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.
- None of the Four Floors of Fear use karate except when Sandy fights Master Udon.
- Near the end of the episode, when Master Udon falls into some water, Sandy says, "You're in the soup now, Udon!" Udon is actually named after a type of noodle soup commonly served in Japan.
- For years, there were rumors that in the original airing of this episode, there was a small amount of blood on Master Udon's face after getting beaten up by Sandy. Vincent Waller on Twitter eventually debunked this in 2018.
- This is the second episode Sandy wears her bikini in public. The first is in "Procrastination."
- This episode is dedicated to Pat Morita, the voice of Master Udon, who died on November 24, 2005, due to kidney failure.
- Pat Morita was previously cast as Mike Woo in the Nickelodeon series The Mystery Files of Shelby Woo.
- This is the second episode to have a dedication for another person, after "Graveyard Shift" (which was in dedication to Max Schreck, the portrayer of Nosferatu); the third and fourth would be "Snooze You Lose" and "Krusty Katering," (dedicated to Barry Anthony) respectively.
- Although this episode was copyrighted in 2006, archival recordings of Pat Morita's voice that he provided as Master Udon are used in this episode.
- Although Pat Morita voiced Master Udon for the most part, the line "I am Master Udon" was provided by Tom Kenny. Kenny also provides Udon's screams and grunts during his fight against Sandy.
- It is unknown how SpongeBob manages to get out of the chair that he was cuffed to.
- In the video game SpongeBob's Atlantis SquarePantis, during the SpongeBob's Director's Commentary for the level "Squidward Super Shooter!," SpongeBob mentions Sandy fighting against Master Udon. Specifically, the first exercise Sandy performed in her exercise program was traveling back to Karate Island and sparring with Master Udon.
- There is a video game based on this episode from the LeapFrog Didj and LeapFrog Leapster Explorer named Fists of Foam.
- According to The Ultimate SpongeBob SpongeBash marathon's Celebrity Favorites, Jennette McCurdy rated this episode as her favorite.
- In the German dub, this episode is called "Der König des Karate," which translates to "The King of Karate."
- The painting of a boat inside Gary's shell is a replica of the painting over the Simpson family's couch.
- This episode contains many references to the films Game of Death, Enter the Dragon, 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. SpongeBob and Sandy being invited to an island is also a similar plot to the one in Enter the Dragon.
- Sandy's and Udon's muscle growth is loosely based on Toguro's 100% muscle growth in the manga and anime series Yu Yu Hakusho.
- Two hand signal parodies are seen in this episode:
- Whenever The Tickler talks, his voice is out of sync with the animation. However, this may not be an error, but rather a reference to the fact that old Japanese films (like martial arts films, which this episode is a stylistic parody of) would have out of sync speech when dubbed into English.
- In one scene, The Tickler has white pants instead of brown pants.
- 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.
- When Sandy kicks Master Udon in the face, she is shown with five toes, but she usually only has three.
- In the Greek dub, Squidward says, "Είμαι εδώ! Είμαι ο βασιλιάς του Καράτε και είμαι εδώ!" which means "I'm here! I'm the king of Karate and I'm here!" but he should say, "Κλαρινέτου" (Clarinet) instead of "Καράτε" (Karate)