Class Clown is the thirteenth episode of Season 4.
Timmy tries to win the heart of Trixie by wishing that he was the funniest guy on Earth but when he finds out that a seemingly harmless plant he gave her is going to grow to gigantic proportions and eat her at 12 midnight. He cannot un-wish his wish either because Cosmo and Wanda are too busy laughing at Timmy to take him seriously.
This synopsis needs improvement.
After a visit to the planet of seemingly harmless plants to get a flower for Trixie, Timmy discovers that Trixie will date anyone who is funny. Timmy wishes that he was the funniest man on Earth and soon, he can make anyone bust a gut just by talking to them. Timmy finally gets Trixie's attention long enough to give her the flower and ask her out. Unfortunately the plant is actually an alien monster. Timmy goes to Trixie's mansion for their date, but she thinks he's joking due to the wish and won't let him in. Later, Timmy learns about the plant from Cosmo who says the Plant will eat Trixie's 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 funny with the rest of Fairy World. Once there, Timmy wishes he wasn't funny anymore. He goes to Trixie's house armed with garden tools to kill the plant.
The plant sprays Trixie with knock-out gas before she sees it. Timmy rushes into Trixie's room to discover that the plant has grown to gigantic size. After a fierce brawl with the plant that nearly destroys Trixie's room, the plant attempts to eat Timmy but he sprays it in the trap with plant-killing gas which suffocates it long enough for Timmy to grab his hedge clippers and cut its stem, thus killing it. 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 lost his chance to be Trixie's boyfriend, he's just happy that she's safe. Cosmo tries to cheer Timmy up by taking them to a the planet with almost enough oxygen but Timmy and Wanda suffocate while Cosmo, somehow still breathing, ponders what air is.