Timmy wishes he had the body of a 16 year old to get on a roller coaster ride. Vicky soon falls in love with him, not knowing that it's really Timmy. When Vicky asks what his name is, Timmy yells out "Gah!" in surprise. He's then known as Gah, the Norwegian male supermodel. Timmy must now find a way to get himself back to his regular age before things go too far.
The Turner family is going to Adrenaland, and they left early to get a good parking space. Chester and A.J. are also along for the ride. Everyone is fast asleep, even Mr. Turner who is driving while asleep at the wheel and mumbling about Dinkleberg. Mr. Turner presses down onto the gas and the car speeds through a construction site, a guillotine factory, narrowly avoiding aliens and a purple Incredible Hulk look-a-like at Area 51, and then over a cliff and directly into the middle of the Adrenaland amusement park, the car landing right ontop of the Dinklebergs. Everyone cheers and exits the car, Mr. Turner tells Timmy and his friends to meet them back at the car at five o'clock, and all three are gone as soon as he finishes his sentence. Mrs. Turner remarks Timmy never moves that fast when he needs to do his chores.
A nearby attendant gets Mr. and Mrs. Turner's attention by bringing them over to a Missing Child Booth. Mrs. Turner remarks that their child isn't missing, and the attendants yell "And that's the password!" and pull a lever, plunging Timmy's parents through a hole in the ground and onto a sofa that moves along a rail, bringing them into Adult Land, which Mr. Turner greets as a "kid-less adult paradise", to which the attendants tell them that should be their slogan. Mr. and Mrs. Turner say cheers and toast their drinks. Back inside the amusement park, a park attendant is letting elderly/high-blood pressure people onto the biggest roller coaster in the park with no problem, but when Timmy and his friends try to get on, they are quickly stopped and turned away when they fail to meet the height requirement. The park attendant tells them they should try the Snail-Go-Round or It's a Dull World, boring rides for little kids.
Timmy yells "This reeks!" while inside Adult Land, his father yells "This so does not reek!" which an attendant tells him makes another great slogan. Mr. and Mrs. Turner are getting facials and manicures, and have forgotten all about Timmy. At the roller coaster, Timmy tries to meet the height requirement with plastic cups on his feet, but he still is nowhere close to being tall enough and falls flat on his face. A.J. makes a calculation and says that none of them will be tall enough to ride the Heartstopper until they are sixteen years old. Chester brought an emergency tall kid kit, a trench coat and mustache which he uses to make him and A.J. tall and adult-like with A.J. standing on his shoulders. The park attendant that took Timmy's parents away earlier comes to Chester/A.J. and thinks he is a real adult and "lost his kid", and hen A.J. says not, he and Chester get sent down in the hole into Adult Land, leaving Timmy alone with no one to be miserable with.
Cosmo and Wanda appear disguised as park attendants wearing organ suits, Cosmo is a kidney, and Wanda is a pancreas. Timmy says he does not want to wait until puberty to ride the Heartstopper, so he wishes he had the body of a sixteen year old. The wish makes Timmy older and into a buff supermodel (because he still has the brain of a ten year old), and he runs off to ride the Heartstopper. Some kids come over and cheer at Cosmo's costume, while scoffing Wanda because the pancreas is not even a vital organ. Chester and A.J. land in Adult Land next to Timmy's parents at a buffet, and A.J. reminds them that they need to meet with Timmy at five o'clock before dropping his mustache and nearly giving himself away when Chester reaches out and grabs it. At the Heartstopper rollercoaster, Timmy makes his way through the line and gets a seat easily because the attendant needs a single rider.
Timmy takes a seat in the roller coaster cart and the female rider next to him starts flirting with him, he yells "GAH!!" when he realizes it's Vicky asking for his name. Vicky asks "What kind of name is Gah?" and Timmy thinks fast and replies its Norwegian. Vicky finds him hunky and thinks he is a Norwegian super model, the top two requirements for her dream man. The ride starts and Vicky tells Gah to hang onto her tight, which he replies in disgust "Why would I hang onto you?", so Vicky takes out scissors and cuts his safety restraints as the cart reaches the top of the roller coaster, and he has to grab a hold of Vicky to survive as the roller coaster plunges back into the Earth's atmosphere. The cart reaches an abrupt stop at the end, and two shock paddles appear and shock Timmy/Gah and Vicky. Gah runs off while Vicky pledges to follow him until she founds someone richer and better looking.
Timmy/Gah runs into the hall of mirrors while Vicky laughs hysterically and chases him. She watches Gah change shape on the mirrors to resemble Timmy and says "Buck tooth and pink hat on a kid? Eww. Buck teeth and pink hat on a Norwegian male supermodel? "Yay!". Timmy runs up to a kidney and pancreas and yells that he doesn't want to be sixteen anymore, but it isn't Cosmo and Wanda in the suits, it is another park attendant whole remarks that at least Timmy is a super model and not a guy in a costume. Vicky catches up with him, and pulls Gah off to the bumper cars. Another girl comes up to Timmy/Gah to get an autograph for her Gah calender, and Timmy remarks "I have a calender?". Vicky smashes the girl away, yelling "He's MINE!". Timmy crashes the bumper car through the wall and drives off, returning a second to sign the girl's calender. Back inside Adult Land, Mr. and Mrs. Turner are enjoying a mud bath, while Chester/A.J. are lying on a bed getting a massage. The attendant is beating on Chester's head and says that he has quite a lump and should take off his coat. A.J. stutters and then his mustache falls off again, the park attendants suspect him and pull the coat off, revealing Chester and A.J. The adults scream in surprise while the kids run off. Two security guards appear and tell the adults to go back into the park and find their kids. The adults groan at first, until the guards tell them that Gah is inside the park which causes much excitement.
Vicky is still searching for Gah, who is hiding inside a trash can. Timmy realizes he has to find Cosmo and Wanda quickly since it is almost five o'clock and he must meet up with his parents soon. Timmy finds Cosmo and Wanda, but they cannot change him back in public and with Vicky chasing him, so Timmy has an idea. He and Vicky go into the tunnel of love, and Timmy must brave a smooch from Vicky who screams in disgust when Timmy is changed back to normal while inside the tunnel, and she realizes she was kissing Timmy. Vicky demands to know where Gah is, and Timmy says that he went back to "Norwegia" (Norway). Vicky gets a snow sled with dogs and arctic gears and sets out to "Norwegia" in search of Gah. Chester and A.J. are cornered by security in Adult Land, and told that they can never speak of this place, because if they do: They won't get to keep spring-pulled talking Gah dolls. The guard pulls the string and the Gah doll says "I have a calender?". Chester and A.J. eagerly accept the deal and the dolls. They are sent back into the amusement park. Timmy, his friends, and parents meet up back at their car, Chester and A.J. are swooning over their Gah dolls, Mr. and Mrs. Turner are upset they missed meeting Gah. Wanda and Cosmo appear as floating organ balloons, Wanda is about to tell Timmy that she hoped he learned his lesson and Timmy interrupts her saying that he actually had a good day, he got to experience puberty, ride on the best roller coaster, and he got rid of Vicky. Timmy then wonders where Vicky is.
Vicky is on her sled in an arctic region (probably Norway), crying that she lost her super model, when a polar bear appears and tells her "And that's the password!" before sending her down into a hole in the ground via a lever.