SpongeBob is watching his favorite show, The Adventures of Mermaid Man and Barnacle Boy. He has entered a competition, and is listening eagerly to see if he's won the mystery prize. The announcer declares the winner as "SpringBoob SquirePin", for his life-sized Krabby Patty mannequins of Mermaid Man and Barnacle Boy. SpongeBob calls out that SpringBoob SquirePin stole his idea, before the announcer announces that he has made a "slight" error in pronunciation, and that the real winner is indeed SpongeBob SquarePants.
Within seconds, SpongeBob's prize is delivered to him. SpongeBob has won the "conch signal", a conch whose noise is a signal which Mermaid Man and Barnacle Boy respond to. SpongeBob blows it, and Mermaid Man and Barnacle Boy hear it from Shady Shoals, and spring into action. When they arrive at SpongeBob's house, they at first think that he has been disfigured before recognizing him. He explains to them that he won the Conch Signal in a contest, and that there is no danger. They then explain to him the importance of the signal, and tell him that he must only use it for major emergencies.
Later on, Mermaid Man and Barnacle Boy hear the conch once again, and rush to SpongeBob's house, where he appears to be dead on the floor. However, he then reveals that he simply is having trouble opening a jar of mayonnaise. Barnacle Boy is angered by this, but Mermaid Man reminds him that emergencies don't come around as often as they used to, and he tells SpongeBob that he can blow the conch "every once in a while." However, SpongeBob abuses this privilege, constantly calling Mermaid Man and Barnacle Boy to help with everyday tasks. Eventually, the two heroes are exhausted, and Barnacle Boy attempts to destroy the conch signal, but cannot bring himself to do so.
SpongeBob apologizes and explains that he just wanted to spend time with them. As a result, they let SpongeBob join them on their daily patrol, with SpongeBob dressed as a superhero. They do various activities, such as keeping lookout from on top of a tall building, and in the street, Mermaid Man and Barnacle Boy see an old man, who was once The Atomic Flounder. SpongeBob attacks the old man, not understanding that he is no longer a villain, and angered by this, the Atomic Flounder breathes fire in Barnacle Boy's face.
At a restaurant, Mermaid Man and Barnacle Boy ask SpongeBob to find their theme song on a jukebox. As he tries to find it, they decide to "ditch" SpongeBob. Outside, however, they are attacked and trapped by their arch nemesis, The Dirty Bubble. When SpongeBob finally finds the song, he comes outside to tell them, and sees their predicament. He asks the Dirty Bubble for his autograph, saying that he is his favorite supervillain, and "accidentally" pops the Dirty Bubble with a pencil tip, saving the day. The trio ride off in the Invisible Boatmobile, singing a Mermaid Man and Barnacle Boy-themed version of "Jingle Bells" as the episode ends.
This is the only "Mermaid Man and Barnacle Boy" where the title card has purple scales. The other four have blue scales on the title card, and "Mermaid Man & Barnacle Boy VI: The Motion Picture" has a completely different title card then the others.