Timmy wants to impress Trixie. He sees that humor impresses her so he wishes to become the funniest person on Earth. The problem with that is nobody is taking him seriously. His parents do not even give him lunch money which he needs. Also Mr. Crocker treats his words like a joke. To make matters worse, he cannot even wish to cancel this wish because Cosmo and Wanda treat his words like a joke. How will he get his fairies to grant his wishes? How will he protect Trixie from the dangerous plant?
After a visit to the "planet of seemingly-harmless plants," Timmy goes to Trixie's mansion, to give her a plant that he brought from there. Chad and Tad insult Timmy to get a laugh out of Trixie, and to make her want to date them. Watching this display, Timmy realizes that girls seem to love guys with a sense of humor.
While sitting in his room, Timmy asks Wanda why she chose Cosmo considering that Cosmo wasn't very smart or particularly handsome. At that moment, Cosmo pours water on a cat, agitating it and making it attack him. Wanda starts laughing and states that that's why she chose Cosmo, because he was funny. Hearing this, Timmy realizes that the way to get Trixie's attention is to make her laugh, so he wishes that he becomes the funniest kid on Earth. Soon enough, he can make anyone bust a gut laughing just by talking to them. Finally able to make Trixie laugh, Timmy is able to hold her attention long enough to give her The Seemingly Harmless Plant and ask her out.
That night, Timmy goes to Trixie's mansion for their date, but because of Timmy's wish, Trixie cannot stop laughing at him and she thinks that every word that comes out of Timmy's mouth is a joke. Timmy leaves in frustration since no one would really listen to him anymore.
Later on, Timmy learns from Cosmo that the plant he gave Trixie would eat her heart out at midnight. Timmy tries to wish the plant away but Cosmo and Wanda won't take him seriously because he's "the funniest kid on earth". Realizing that the wish only holds true if he is on Earth, Timmy tricks them into going to Fairy World by offering to share his humor with the rest of Fairy World. Once there, nobody thinks Timmy is funny since he wished he was the funniest kid on Earth. He is able to wish himself back to his unfunny self.
Armed with many garden tools to kill the plant, Timmy goes back to Trixie's mansion. In her room, the plant has grown enormously in size and it sprays Trixie with a gas to render her unconscious. After Timmy's fierce brawl with the plant that nearly destroys Trixie's room, he is finally able to grab some hedge clippers and cut its stem in half. The plant quickly shrinks and shrivels up. Eventually, Trixie wakes up in Timmy's arms confused, and when she sees that he killed her plant, she dumps him and kicks him out the window. Even though Timmy missed his chance to be Trixie's boyfriend, he's also happy that she's safe.
Cosmo continues the tour of planets by taking them to the "Planet With Almost Enough Oxygen." Timmy and (surprisingly) Wanda suffocate, because there is "almost enough" oxygen. Cosmo did not suffocate, (apparently because he already took a breath before going, since he knew where they were going or because, like other episodes he and Wanda do not need to breathe (see episode Transparents.)