Timmy gets a quick lesson in “be careful what you wish for” when he wishes to be a grown-up. Rather than becoming the strapping, muscular, older Timmy of his dreams, he turns into a short, bald, hairy-backed, middle-aged man!
While at school, Timmy Turner is picked on by Francis, the school bully, because he is too small. At football tryouts, Timmy's position is that of "being the ball”, due to his size. At home, Timmy's parents are going out to a movie which he is not old enough to see, so Vicky has to babysit him, and she commands Timmy to go to bed at an early time. In his room, Timmy thinks of all of the things that he can do as an adult. He thinks of helping old ladies cross the street, picking on Francis, and not needing a babysitter anymore. Timmy wishes that he was an adult, but after the wish is granted, he looks at himself in the mirror and is disgusted, due to the fact that he was old and lost all his hair. He wishes that he had hair on his head, but Wanda checks Da Rules, which states that fairies can only grant wishes to kids, not adults.
Despite his sub-par appearance, Timmy tries cool adult things. However, he realizes that driving is boring, helping old ladies cross the street as an adult is creepy (due to the fact that children mostly do it for good deeds), adult movies are gross, and shaving is painful. Determined to find an advantage to being an adult, Timmy goes to a restaurant and orders food on the adult menu, but he does not have any money to pay the bill and has to work at the restaurant for the rest of the night. He tries to go home, but not recognizing Timmy as an adult, Vicky kicks him out. Vicky then goes upstairs to check on Timmy and sees that he is gone, much to her shock. When Timmy's parents come home, Vicky fools them by putting a pink hat on a basketball on the pillow of Timmy's bed. Mr. Turner wishes his basketball-shaped son goodnight, while Vicky leaves to find Timmy before he gets hurt, or worse, she gets in trouble.
Banned from his own home, Timmy goes to the football field and sleeps in the bleachers. The next day, Timmy tries to stop Francis from bullying Chester and A.J. by plucking him in the head. However, Francis cries crocodile tears in front of the police when he sees them roll by, and the police go after Timmy for "abusing" a child. They chase Timmy down, until Cosmo and Wanda hide him from the police. Afterward, a reassigning fairy declares that Cosmo and Wanda have to be re-assigned to a mean and scary kid since Timmy is no longer a kid. Depressed, Timmy bribes the fairy with five dollars so he can spend two more hours with Cosmo and Wanda. Timmy has an idea: maybe if he acts like a kid, he can become a kid again.
Timmy tries various childhood activities, such as mini-golf and riding kiddy rides at the grocery store, but none of them seem to work. Instead, because of his creepy activity, Timmy gets arrested. The reassignment fairy comes back and says that someone has to be assigned to the new kid, but Timmy cries and yells that he wants his mom and dad. Watching Timmy's tantrum, Wanda sticks a thermometer in Timmy's ear to measure his childishness. According to the thermometer, Timmy is becoming a kid again, and Wanda explains that Timmy wanting his mommy and daddy was making him have the mind of a kid again. Since Cosmo and Wanda are able to listen to him now, Timmy wishes that he was a kid again. The wish works and the reassigning fairy ends up getting reassigned to the freaky kid. Timmy gets out of jail and goes home with Vicky, who was conveniently at the jail cell filling in a lost child report.
When he goes back to school, Francis is bullying Timmy again, but Timmy, who is no hurry to grow up, is not bothered by Francis' jokes and realizes that being a kid is great, especially since he still has all of his hair and none of it's on his back. After seeing Francis' back hair, Timmy gives him a razor and shaving cream with the episode ending with Francis thinking shaving will be fun as he goes off, but he ends up experiencing the same pain as Timmy did earlier. Timmy and his godparents then laugh their heads off as Francis screams in pain offscreen.