- Purple baby (debut)
- Barnacle Boy (debut)
- Cafeteria workers (debut)
- Clam Head Candy Cad (debut)
- Incidental 89 (debut)
- Hammerhead Shark
- King Neptune (mentioned)
- Man Ray (mentioned)
- Mermaid Man (debut)
- Incidental 88 (debut)
- Old Man Walker (debut)
- Patrick Star
- Thief (debut)
- Barnacle Star (debut)
- Reflecto (mentioned)
- Shady Shoals caretaker (debut)
- Sea urchins
- SpongeBob SquarePants
- Purse lady (debut)
- Mermaid Pants (debut)
- Squidward Tentacles
- The Chief (mentioned)
- TV Narrator
- Incidental 81 (debut)
- Incidental 82 (debut)
- Incidental 84 (debut)
- Incidental 85 (debut)
- Incidental 86 (debut)
- Incidental 90 (debut)
- Incidental 91 (debut)
- Incidental 92 (debut)
- Incidental 93 (debut)
- Incidental 94 (debut)
SpongeBob and Patrick look at the journey of the famous superheroes - Mermaid Man and Barnacle Boy. They decide to visit Squidward, who is annoyed by the antics of his friends. By calling Squidward Reflecto, SpongeBob and Patrick learn that their characters actually live in a nursing home and have long since retired. They decide that such crime fighters have nothing to do in retirement, and come to the Shady Shoals nursing home, where they are met by its caretaker. He escorts the friends to the superheroes. SpongeBob tells them that superheroes should not be retired, but they should fight evil. Mermaid Man creates a panic when he hears the word "evil." Meanwhile, the caretaker of the nursing home arrives and throws SpongeBob and Patrick out.
While the evil fighters are going to the cafeteria for lunch, SpongeBob arrives and makes an inspiring speech about how superheroes shouldn't be retired. While SpongeBob is telling this, the caretaker comes again and throws him right out to his house.
SpongeBob and Patrick come up with a new plan to free the superheroes from retirement: SpongeBob, disguised as a woman, drops his bag, and Patrick, disguised as a criminal, steals it. Suddenly, Barnacle Boy comes to the Sponge and says that they will always be retired, because there is no evil. Mermaid Man interrupts him and replies that there is always evil. They start arguing until SpongeBob interrupts them and says that he and Patrick have painted their invisible boatmobile. Barnacle Boy decides to end the argument and rubs Mermaid Man that evil is right in front of them. It shows their rings, which resume their superhero activities, and the evil fighters get into a fight with SpongeBob and Patrick, who are admired from there. Mermaid Man and Barnacle Boy do everything possible to remove unwanted guests: Waterballs, Raging Whirlpool. And finally, for the third time, they are lucky: they are helped by elderly residents of a nursing home who pick up SpongeBob and Patrick and push them back into their homes.
Eventually, a show of Mermaid Man and Barnacle Boy is created: New adventures of Mermaid Man and Barnacle Boy. The Duo play checkers, then suddenly the phone rings. Mermaid Man picks up the phone and thinks the phone is broken and hangs up. And then the voice on the TV says: "Will our heroes ever get their phone fixed? Tune in next week and find out." And when SpongeBob winks, the words appear: "Wink!"
- Mermaid Man screaming "Evil!"
- SpongeBob and Barnacle Boy saying "There's evil afoot!" and the winking old man.
|( ‣ ) Associated production music|
|( • ) Original music|
|( ◦ ) SpongeBob music|
‣ The Lineman - Sam Spence [Title card]
‣ Arizona Fanfare - Jack Shaindlin [opening]
‣ The Adventure Begins - Jonathan Neal [SpongeBob and Patrick dressed as Mermaid Man and Barnacle Boy]
‣ Enter the Villain - Chris Payne, Paul Rogers ["Reflecto, up to no good!"]
‣ Super Hero - Tony Hymas ["Donuts!"]
‣ Call to Destiny (a) - Ron Goodwin ["Creatures of the deep, assemble!"]
‣ Police Car - Sam Spence [a jellyfish appears]
‣ Death Trap [#25] - Gregor F. Narholz ["Reflecto has found our secret lab!"]
‣ Super Hero - Tony Hymas ["Mermaid Man and Barnacle Boy reunite!"]
‣ Hawaiian Link (b) - Richard Myhill [rest home exterior]
• Gator - Steve Belfer [Mermaid Man, Barnacle Boy watching TV]
‣ War Agitato - Jack Shaindlin [Mermaid Man panics/Mermaid Man panics again]
‣ Hymn of Achievement [#7.03] - Gregor F. Narholz [SpongeBob's speech about old people]
‣ Queen's Aloha Oe B - Kapono Beamer, Queen Lili'uokalani [lunch]
• You're Nice - Sage Guyton, Jeremy Wakefield ["Do you remember when the food supply in Atlantis was running low?"]
‣ Wedding Procession - Felix Mendelssohn-Bartholdy, Leslie Pearson [SpongeBob kicked out]
‣ Hawaiian Link (a) - Richard Myhill [later...]
‣ Sweet Annabelle - Sam Fonteyn [SpongeBob dressed as a lady]
‣ Blackbeard - Sam Fonteyn [Patrick dressed as a burglar]
‣ Blood in the Gutter - Laurie Johnson [Mermaid Man sleeping with eyes open]
• SpongeBob Closing Theme - Steve Belfer, Nicolas Carr [painting the Invisible Boatmobile]
‣ War Agitato - Jack Shaindlin ["Evil!"]
‣ Arizona Fanfare - Jack Shaindlin [fanfare]
‣ The Lineman - Sam Spence [water balls/"Sea creatures, attack!"]
‣ Moloka'i Nui (a) - Kapono Beamer ["I feel five years younger."]
‣ Arizona Fanfare - Jack Shaindlin [SpongeBob and Patrick watching the new show]
‣ Caped Crusader - David Farnon [Mermaid Man and Barnacle Boy playing checkers]
‣ Police Car - Sam Spence [sting at end]
- This episode is available on the Tales from the Deep, 10 Happiest Moments, Absorbing Favorites, The Adventures of SpongeBob SquarePants, Complete 1st Season, From the Beginning, Pt.1, Mermaid Man & Barnacle Boy: Defenders of the Deep, SpongeBob SquarePants, iSponge Offer Pack, From the Beginning, Pt.1, My house, My pineapple, SpongeBob SquarePants Vol. 1, Lost at Sea (Polish only), First 100 Episodes, The First & Second Seasons, Movies & TV Collection, The Best 200 Episodes Ever, and Hall Monitor DVDs, along with SpongeBob SquarePants Volume 2 on GBA Video.
- This episode is also available on the Deep Sea Sillies, Absorbing Favorites, and Tales from the Deep VHS tapes.
- "Mermaid Man and Barnacle Boy" was ranked #37 during the Best Day Ever event from November 9–10, 2006.
- Currently retired critic Paul Pritchard of the DVD Verdict, in a review of 10 Happiest Moments, said that this episode "stood out" for him from the rest, by calling it "the perfect encapsulation of everything that makes SpongeBob great." He also praised Mermaid Man for getting "some of the funniest lines in SpongeBob history."
- This episode marks the first appearance of Mermaid Man and Barnacle Boy (hence the title).
- This episode marks the first time SpongeBob is seen in drag.
- Mermaid Man and Barnacle Boy have five fingers as youngsters, though as elders, they have four.
- When SpongeBob and Patrick are first asking the man at the front desk to see Mermaid Man and Barnacle Boy, an article on the inside of his newspaper reads, "School Drop Out Rate Soars." This has a double meaning because "school" is also the name of a group of fish.
- This is also referenced in "Spin the Bottle."
- The old captain is played by Don Newhouse, the man who guarded the gates at Nickelodeon Animation Studio in Burbank, California at the time. He later reappeared in "Sun Bleached" and is also the inspiration for the character "Old Kid" from Invader Zim.
- When Mermaid Man picks up the phone during the game of checkers, the phone's ringtone is the same sound effect used for number 11 of the 13 bad words in "Sailor Mouth."
- This is the only episode in which the Mermalair is called the Sea Cave.
- This is the first episode of the Mermaid Man and Barnacle Boy series.
- This is the first episode to use music from the NFL, "The Lineman," which was used in most of the Mermaid Man and Barnacle Boy episodes.
- Before SpongeBob and Patrick are watching TV, when Mermaid Man talks with Barnacle Boy, there are two Old Man Jenkins, green and blue.
- This episode marks the first appearance of Shady Shoals Rest Home.
- This episode contains the first mention of Man Ray, although never appeared physically until "Mermaid Man and Barnacle Boy III."
- During the Mermaid Man-a-Thon, this episode was paired up with "Mermaid Man and Barnacle Boy II."
- The sequels to this episode used the same title card art until "Mermaid Man & Barnacle Boy VI: The Motion Picture."
- This episode, along with "Pickles," premiered on Stephen Hillenburg's 38th birthday.
- This is Mark O'Hare's first episode as a writer.
- Atlantis is first mentioned in this episode before making its first actual appearance in the episode "Neptune's Spatula."
- Mermaid Man and Barnacle Boy are parodies of both Aquaman and Aqualad and Batman and Robin.
- The Adventures of Mermaid Man and Barnacle Boy is a parody of the Batman TV show from the 1960s as well as that of Filmation's Aquaman animated series. The title for MM&BB is also similar to the Aquaman title card, with the whirlpool and all.
- Young Mermaid Man here looks almost exactly like Aquaman, except with a star on his nose, two seashells on his shirt and his belt buckle is shaped like an M instead of an A.
- The Aquaphone is a parody of the Bat Phone from Batman.
- The invisible boatmobile is a parody of both Wonder Woman's Invisible Jet and the Batmobile.
- Reflecto is a parody of Electro and Birdman villain Reducto.
- The Sea Cave is a reference to the Batcave.
- When Patrick takes the doughnut from the box, it is orange, but when they "unite," it is replaced with the chocolate one which was left in the box. After that scene, it changes to the normal doughnut again.
- Then when they reunite again, it goes back to chocolate.
- After Patrick takes SpongeBob's purse and peers out from behind the building, the purse is seen floating beside him.
- Barnacle Boy puts the wrong ring on Mermaid Man's hand.
- After Mermaid Man says the oath, both his and Barnacle Boy's rings disappear.
- When Mermaid Man and Barnacle Boy say "Dog paddle, away!" and start dog paddling, Mermaid Man's socks are missing. Afterward, they return.
- When Mermaid Man says "Up, up, and away" while rocking, Barnacle Boy appears to be saying it too, but with no voice.
- When Barnacle Boy shouts, "It's not me, old coot," the sclerae on both his eyes turn blue.
- In this episode, Mermaid Man and Barnacle Boy are revealed to have come out of retirement but in later episodes, they seem to be still staying in Shady Shoals, such as in "Night Light," "Mermaid Man & Barnacle Boy VI: The Motion Picture," and "Shuffleboarding."
- When Mermaid Man says "To the broccoli!", his right bra strap turns orange for a split second.
- After Mermaid Man and Barnacle Boy say "Dog Paddle away!", Mermaid Man is not wearing tights.
- When Mable comes outside after Mermaid Man says, "sea creatures assemble," the bottom of her cane flashes white for two different frames.
- ^ Pritchard, Paul (2010-09-19). Review - SpongeBob Squarepants: 10 Happiest Moments. DVD Verdict. Archived from the original on 2013-09-22. Retrieved on 2019-07-15.