Timmy wishes that everyone ate nothing but dessert, which at first leads to an increase in energy. However, over time, everyone becomes obese, which Timmy sees as a partly good thing: no one is made fun of for being fat, and Francis tries to pick on thin kids (Mark Chang) but is too fat to even move! But the huge increase in weight is causing the Earth to crash into the Sun! But Wanda doesn't have the energy to lift her arm to grant the wish!
During dinnertime, Timmy doesn't want to eat his spinach, so he throws it away, leaving Cosmo and Wanda to vaporize it, but when he asks for dessert, his mom gives him carrot cake--without the cake, because Mr. Turner says that carrot cake minus cake is healthy for their colons. Timmy then goes to A.J's house for dinner but discovers that for dessert, they read math books. Timmy tries yet again at Mark Chang's place, thinking it would be a better option since he is from Yugopotamia and everything is backwards, so they would serve dessert first. However, since everything is backwards, it turns out the Yugopotamians eat vegetables for dessert.
After his failed attempts for sweets, Timmy gets fed up and wishes for no breakfast, lunch, or dinner; just desserts. Wanda tries to explain that eating healthy gives one the energy to perform tasks, but Timmy then exploits Wanda's weakness for sweets by talking about chocolate, making her go nuts and beg Timmy to make the wish. Not usually the skeptical one, Cosmo tries to explain what happens to Wanda if she consumes too much sugar, but Wanda puts him in a bubble before he could say what would happen, so Timmy makes the wish.
The next morning, Timmy's Mom wakes Timmy up for dessert and gives him a plate full of sweets. He devours it quickly, while his fairies munch an ice cream sundae in their fishbowl. Pretty soon, everyone gets sugar buzz from eating so much sugar. School only lasts for three minutes since everyone is on an insane sugar rush. Everyone is running marathons and everything is hurried. Later, Cosmo explains that the sugar-induced energy will not last and something else will happen--but he doesn't remember what.
Twenty-eight days later, everybody is morbidly obese, and hooked on sugar. Everyone has lost the sugar rush energy and they become lethargic instead. Timmy's bed snaps in half under his weight. Timmy asks if constant dessert led to them being fat, which Cosmo responds to with a "No that's not it" and eats more chocolate. As Timmy's Dad barges into the room using his stomach flab, Cosmo and Wanda poof into their fish bowl, which can barely contain the two. Timmy's Dad has to roll Timmy down the stairs, Cosmo and Wanda are too heavy for their wings to lift them, causing them to stay in Timmy's room to eat more fatty foods and therefore get even fatter than they already are. The school bus collapses under all the weight when it reaches school. In the school hallways, since everyone is bloated from all the unhealthy desserts, they are forced to roll around like beach balls instead of walking, all except for Mark Chang since the Yugopotamian desserts are vegetables and healthy food. Because Mark is different and fit, Francis tries to stuff him into a locker, but he doesn't have any energy to bully anyone anymore. Which leads Timmy to think that this is a preety good wish along with the fact that no one is made fun of for being fat anymore meaning anyone can get as fat as they please with no consequence. Timmy stuffs his face with another chocolate bar, the bell rings and Timmy rolls off to class.
During class, Mr. Crocker explains what would happen if one area of the earth about the size of Dimmsdale had a massive increase in weight: the Earth hurtles into the sun. Then it's snack time. The kids cheer and open their desks, which are filled with sweets. They eat more desserts, and everyone in the class gets heavier. Trixie eats a giant cookie which causes her belt to snap due to the massive size and weight of her stomach and buttocks. The large bellies cause the chairs to sink to the floor and the Earth begins hurtling toward the sun. Everybody in the room starts rolling for the exit but gets stuck on the way out. Timmy rolls back to his house but is too unhealthy and fat to climb the stairs. He then does a butt slam which causes the ceiling to break and his FattyGod Parents to come down to him. He tries to wish everything back to normal, but Cosmo lost his wand somewhere in his layers of fat and Wanda does not have enough energy to lift her arms. When Timmy tries to lift her arms for her they only get sunk in her fat. All the force Timmy gives is nulified and absorbed by the fat in her arms. He realizes she was right that we need healthy food to have the energy to do things like lifting your arms.
Remembering that Yugopotamians eat healthy food for dessert, Timmy goes to Mark Chang's spaceship and tries to get his last can of spinach cobbler. At first, Mark Chang declines, but after Timmy sits on him, he lets him have the can of spinach cobbler, then Timmy ask Mark if he could lift Timmy to the house, but Mark thought he mean it literally and react badly, saying that he can't lift Timmy, and then Timmy said he meant using his ship to lift him, then Mark Chang drops Timmy off (literally) at his house. After landing on Wanda from his fall, Timmy forces the spinach down Wanda's mouth. Wanda then has a brief period of energy, that immediatley goes away as she struggles to lift her arm still but can do it. Timmy uses this to wish everything back to normal. Timmy learns his lesson of eating healthy food, and it is seen that Cosmo still has his extra weight. Wanda is then revealed to be stuck somewhere in Cosmo's layer of fat, and as he tries to retrieve her, he only finds two cowboys who appear to be chasing each other