When the popular game show ""Who Wants to Win a Bunch of Money?"" comes to the town of Onion Falls it seems like the perfect oppurtunity for straight-A student Keo to use his knowledge to win some cash. Unfortunately, Keo is terrified of appearing on television. Keo's best friend, aspiring stand-up comedian Yakkity, dreams of being on TV and becoming famous so the two decide to team-up by having Yakkity go on the show with Keo hiding in his hat. Yakkity becomes a sensation but Keo doesn't like it when people start to think that Yakkity is smarter than he is.
Yakkity finds out that his grandfather Yardly Yak was a champion grass-eater who was cheated out of a first place trophy by a rival known as The Cowinator. Granny persuades Yakkity to challenge Cowinator to a grass-eating contest and restore the family name. The Cowinator accepts the challenge but because he's too old to compete he forces his grandson Cowinator III to compete for him.
Professor Crazyhair introduces Penelope, a robot daughter he created to assist him in his lab. When Penelope displays a lack of social skills Lemony decides to teach her how to act like a regular teenage girl. Soon Penelope is shopping at the mall, getting her hair done and throwing temper tantrums. Penelope becomes so much like a real teen that she ends up becoming the most popular girl at school which wouldn't be a problem except that the football team wants her to replace Yakkity as the school's mascot.
Keo's dad has plans to compete with his son in Onion Fall's annual picnic so that they can beat Yakkity and Mr. Highpants, who were the previous year's winning team. Keo fears that his father will only embarass them since his father is only a pineapple without a body so Yakkity decides to help him out. He asks Professor Crazyhair to build a robotic body for Keo's dad and all seems to go well until the overly-competetive robot begins to think for itself.
After scoring high grades on a test at school Yakkity, Keo and Penelope are invited to a special camp. Yakkity is excited about going to camp with his friends until he finds out that the camp is actually for smart kids who study science and technology where the activities include programming commands for a giant robot.
Yakkity's manager Mr. Trilobyte loses his office after he ends up going broke because he spent all his money on sweepstakes tickets. When Yakkity drops by to pay Mr. Trilobyte the 10% he owes him from his last stand-up comedy gig Trilobyte gets an idea. He decides to live off of Yakkity by telling him that he doesn't just owe him 10% of the money he earns on jobs but he also owes him 10% of everything he currently owns.
The kids decide to work on a movie for the Onion Falls Video Fest but they can't agree on what kind of movie to do so they split up into groups. Yakkity and Lemony decide to work on a comedy about hidden camera pranks while Keo and Penelope work on a sci-fi B-movie about a girl from the future with a warning about a mutant onion.
Keo and Lemony worry after Yakkity fails to show up and perform at a kids' birthday party and at the school's football game. When they look for Yakkity they find him on top of the water tower with a giant pimple. They discover that Yakkity's strange behaviour is the result of some changes that yaks experience when they get older such as breaking out, constant sweating and the urge to reach higher, cooler areas to escape from the heat.
Yakkity learns hypnotism and decides to use it to get people to laugh at his comedy but his plan backfires when the whole town laughs at every single thing that he says.
Yakkity decides to add a ventriloquism dummy to his comedy act but it only becomes a hit when a nasty worm inhabits the puppet and starts doing the talking.
Yakkity, seeking some freedom, decides to srike out on his own and move into a nearby treehouse. But when Granny rents out Yakkity's old room he soon becomes suspicious of the new tenant.
After breaking into Professor Crazyhair's lab and causing a mess Yakkity and Keo are forced to do some house cleaning as their punishment. While cleaning, the boys end up in an even bigger mess when they lose one of the professor's more important inventions.
After Yakkity worries Granny by coming home late, she decides to keep track of him by giving him a large mobile phone. Meanwhile, Rondo tries to figure out a way to win a fighting robots competition.
Yakkity is charged a fine when the library realizes that he hasn't returned a book called ""Darn-it Dog"" which he checked out over 10 years ago. Rather than pay the fine or give back his favorite book, Yakkity lies and claims that he never checked out the book. Eventually Yakkity begins to lie about everything to get what he wants.
In order to keep from losing his position as one of Mr. Trilo's clients, Yakkity promises the agent that he can organize a huge entertainment event called Yakstravaganza. And to pull it all off Yakkity seeks the help of former entertainment legend Keo's Dad.
A big hair mistake leads Yakkity to don a bad perm. He tries to make the best of the situation until the hair style begins to take on a life all its own.
After a football accident leaves Yakkity's antlers bent out of shape Granny tries to get them fixed by getting him antler braces. Yakkity then becomes the most popular guy in town when his braces begin to act as a satellite that picks up all sorts of great television channels.
Keo tries to impress Lemony by getting over his fear of public speaking and running for class president. But Keo's plans to improve the school don't turn out as expected thanks to some support from campaign manager Yakkity.
Yakkity winds up on TV and unwittingly predicts the weather correctly. Impressed with the correct prediction, the TV station decides to make Yakkity it's new weather yak unaware that his prediction came to him courtesy of Granny's foot boils.
Yakkity and Keo fear the worst when Keo's Dad is nowhere to be found. The two search for the missing pineapple only to find themselves with no clues and an entire town full of strangely suspicious residents.
Yakkity becomes jealous when Keo gets a new pet so he decides to order his own pet and what he gets is a snakehead fish that can live outside of water. Yakkity grows close to the fish but Keo warns that the snakehead fish is a wild animal that shouldn't be kept captive.
Yakkity's irresponsible scooter driving causes an accident which leaves Chuck Damage with amnesia. Lemony tries to help Chuck but Yakkity worries that she'll help the wrestler remember his sensetive side instead of his destructive side.
Yakkity, Keo and Lemony are teamed together on a school project designed to help the town smell better. Lemony completely takes over the project when she decides to make use of some discarded oil by turning into a new lemon-scented fragrance. Lemony is praised and awarded for her unique creation until Yakkity and Keo discover that the oil in the perfume can turn people into human balloons.
Yakkity is embarassed by a new hat that Granny made for him but eventually the hat becomes a popular fashion trend and Yakkity is credited as the creator. When the hat frenzy dies down Yakkity partners up with Keo and Mr. Highpants so that they can come up with the next fashion trend that will make them rich.
Yakkity decides to build a float of his own for the annual parade when the town fails to recognize his comedic hero.
Trilo has Yakkity fill in for Wanda at a children's birthday party. When Rondo catches sight of Yakkity, dressed up as Wanda, he instantly falls for for the yak and invites him to a party as his date.
Yakkity begins to believe that he's special when he discovers that yaks are endangered and everyone starts treating him differently.
Yakkity and Keo come up with their very own sport and name it Yakeo which becomes popular with everyone except football jock Rondo.
After firing Mr. Trilo, Yakkity joins the sneaky Max Dewey and his circus.