SpongeBob is shown at the Krusty Krab during closing time with Mr. Krabs and Squidward. He is in the kitchen and starts singing a song. Squidward gets annoyed because SpongeBob is singing that song. Mr. Krabs comes and says to mop the floor. Squidward tells SpongeBob again to stop singing. Squidward then puts on the radio, but it's playing "Musical Doodle" - SpongeBob picks up the radio and starts dancing and singing to it. After it ends, SpongeBob sighs happily. Next, Mr. Krabs says it's closing time again and Squidward runs out saying "Freedom!" SpongeBob is sitting down on a table singing to the song. Mr. Krabs yells "SpongeBob!" SpongeBob floats out of the Krusty Krab and lands outside a record shop. He hears "Musical Doodle" playing. He then buys the record and goes home, opens up Gary's shell revealing a record player, and plays the song until bed-time. SpongeBob says "Come to bed when you feel like it." He goes to sleep but then starts whistling the song. He goes back downstairs to listen to the song again.
He is then shown at the Krusty Krab the next day, looking very tired. Mr. Krabs warns him to get to work. Mr Krabs hears SpongeBob singing the "Musical Doodle." He is warned to stop singing that song again, but finds it very hard to not sing the song. He hands out Krabby Patties while singing the song, annoying the customers. Mr. Krabs comes out of his office to see what's wrong. All the customers blame SpongeBob for annoying them. Mr. Krabs warns SpongeBob once again to stop singing that song, or he will send him home. In the kitchen, his eyelids form into mouths and start singing the song - the same thing happens with his back.
In the next scene, the kitchen is shown absolutely wrecked, with "doodle" written on the walls and everything else is wrecked as well. Also, there are condiments and patty batter all over the wall. After Mr. Krabs tells SpongeBob to leave, SpongeBob seems that he has completely lost it. He goes home while singing the song - Sandy, Mr. Krabs, Patrick and Squidward follow him, tie him to a chair and gag him. Sandy explains that he has an "Earworm" - An infection in which the person is stuck on a catchy song and is unable to stop singing, the more the person sings the song, the more infected and stronger the infection gets and more crazier the person gets. Mr Krabs wanted to just pull the earworm out but Sandy says that they can't do that or else it will damage his brain and it can only be cured if another song gets stuck in their head. Mr. Krabs sings a sea-shanty and Sandy sings a Western song, but none succeed. Sandy says "someone with musical talent" can cure him (in this case, Squidward). Squidward starts playing his clarinet. The earworm gets annoyed and leaves. Squidward is then shown in bed, praising himself. The earworm comes and goes into Squidward, and now Squidward has the earworm and humming his song. Squidward then screams loudly as the episode ends.
Sandy said they couldn't rip the earworm out, or it would damage SpongeBob's brain. However, she stuck her hand all the way into SpongeBob's head and put a hat on the earworm, nearly touching the brain.
To prevent anyone else from getting infected by the earworm, they could have just squished it before it infected Squidward.
Sandy mentions the episode title when she says "You're suffering from a condition known as Earworm. You see, when your brain becomes stuck on a catchy tune, you're spectable with Earworm infection and you've got a nasty one there.
This episode premiered in the UK four years after "Wigstruck" premiered.
Mr. Krabs' song can also be heard in the Tom and Jerry episode" Jerry's Cousin", although without vocals and slightly higher-pitched.
This episode is similar to, and has the same premise as, the Dexter's Laboratory episode, Head Band. In it, Dee Dee has a song stuck in her head, thanks to a microscopic boy band in her ear. The song spreads to other members of the household, and finally ends when the band breaks up. Earworm is different, in that only SpongeBob gets a song stuck in his head and the culprit is in his brain, not his ear.
The scene where SpongeBob writes letters on the wall is a reference to "The Shining" where Jack (Jack Nicholson) is writing "all work and no play makes Jack a dull boy" on the walls of the hotel.
The ending is similar to the movie "The Exorcist," like when SpongeBob crawled like a spider around his house then inside the window.
"Earworm" is a real-life condition in which a song plays over and over in one's head. Like in the episode, a commonly used cure for earworm is to play another catchy song.
Even though in "Plankton!," SpongeBob's brain is pink, but in this episode, shows it his brain is blue.
Typically, your eardrum prevents you reaching to your brain through your ear alone. Though, it is possible that sponges don't have eardrums (hinted in "Blackened Sponge").