Squidward is relaxing in his bathtub when SpongeBob and Patrick, who are playing with SpongeBob's old baby toys, disturb Squidward, who comes out and demands them to stop making noises and act mature. However, SpongeBob and Patrick do not listen. Squidward walks back to his house, but he slips on a toy along the way and hits his head. His injuries to the head cause him to act like an infant himself. SpongeBob and Patrick then bring him to the hospital.
At the hospital, a doctor tells them that Squidward has developed a mental disorder called "Head-Go-Boom-Boom-Itis" due to his head trauma. It means he is mentally and socially an infant despite being an adult. The doctor also says that his head must be protected from more injury, or else he will likely remain infantile.
However, later at SpongeBob's pineapple, when SpongeBob decides to feed Squidward, Patrick hurts Squidward's head by tossing him up into the ceiling. SpongeBob reminds him that they are supposed to protect Squidward's head, not hurt it more. SpongeBob tries to feed Squidward, but he does not like his food and spits it all over SpongeBob. Patrick licks the food off SpongeBob's face and thanks Squidward for sharing. Then, they try to entertain Squidward with some games. They start off with "Kelpy Cake" (parody of Patty Cake), but when Squidward does it, he ends up slapping SpongeBob and Patrick repeatedly, sending them flying into the air and out of the house into the street.
Squidward escapes, and SpongeBob and Patrick follow him as he crawls out to the street, where a truck is about to hit Squidward. The truck driver is asleep, but Patrick is able to protect Squidward and lift up the road to send the truck flying. However, when Patrick drops the road back into place, he accidentally smashes Squidward's head again. Then, SpongeBob and Patrick go back inside and put Squidward in a cage.
He cries for several days. Then, a few days later, SpongeBob goes to work with Patrick and asks him if he can watch Squidward while he works, but Patrick, tired from taking care of Squidward, falls asleep on a table.
SpongeBob informs an angry Mr. Krabs that Squidward has the mind of an infant and cannot work. Mr. Krabs does not care, since there are customers outside the door, literally begging to give Mr. Krabs their money. SpongeBob then gets an idea. He brings Squidward to the cash register and puts him in a high chair. When Squidward drops his head on the cash register, SpongeBob duct-tapes him to the seat so he cannot escape. When Mr. Krabs lets the customers in, Squidward sucks on a customer's hand and head and SpongeBob stops him.
SpongeBob then gives Squidward the order papers to write orders on, but Squidward instead doodles on them and throws them into the kitchen. SpongeBob asks him if it will be for there or to "goo." SpongeBob peeks out the order window and sees Squidward doodling on Nat Peterson's face. Nat screams and ends up running out of the restaurant. Pilar scolds Squidward for not taking an order. Squidward types in something then he bangs his jaw on the cash register and wails loudly.
SpongeBob thinks Squidward is crying because Pilar yelled at him and SpongeBob tells Pilar he should be ashamed. Squidward then dirties his diaper. The customers smell the horrible stench of Squidward's diaper and Mr. Krabs tells SpongeBob to change it. Everywhere he goes to change Squidward's diaper, people are sitting or getting condiments or walking. Mr. Krabs tells SpongeBob to take Squidward to the kitchen, where the food is prepared, and change it. This disgusts the customers, so they leave.
Mr. Krabs tells SpongeBob to get Squidward out of his restaurant. SpongeBob cries and tells Mr. Krabs that if Squidward is not welcome there, he is not staying either. However, on the way out, he ends up slipping on Squidward's baby rattle and slips into the kitchen, where he bangs Squidward's head on the frying pans and the fryer.
Squidward falls into the ice machine and ice pours out onto Squidward's head, curing his head trauma and returning him to his adult state. SpongeBob is relieved that Squidward is "grown up again" and a confused Squidward asks why he would not be grown up. He notices that he is wearing a diaper and that it is full.
When SpongeBob is about to tell Squidward that he was going to change him, a deadpanned Squidward tells SpongeBob to not say another word about that experience, ever, and then leaves the scene. SpongeBob then replies, "Kids, they grow up so fast" as the episode ends.