Timmy and Poof want to watch a horror movie called, "I Was a Teenage Blowfly", but are forbidden by their parents to watch the movie. They soon get poofed to fly size and a fly's head gets stuck on Timmy's body.
Timmy is planning to watch the 3-D horror movie I Was a Teenage Blowfly, when his parents come in and say that this movie is too scary for him (because it was rated FTTFTT; meaning "Far Too Terrifying For Timmy Turner"). Mr. and Mrs. Turner, then, take Timmy's TV out of the room so he won't be tempted to watch it and so they can watch it on two TV's. Timmy, then, says that's what he has fairy godparents for, and Cosmo and Wanda poof up a widescreen TV. However, it is not for Timmy; it's for his fairies so they can watch the movie in the fishbowl. They tell Timmy to take care of Poof because Poof is, also, too young to watch the movie. While Timmy is changing his godbrother's diaper, he decides to wish himself as a fly on the wall so he can sneak in and watch the movie. Poof does this by (unknown to Timmy) switching Timmy's head with a fly's head. Both "Fly" Timmy and Poof take off across Dimmsdale to try to find a place to watch the movie, resulting in various adventures made more dangerous by Timmy's and Poof's now small size. In the Turner home, "fly headed" Timmy scares Mr. Turner who promptly calls the police. After being nearly consumed by Denzel Crocker's spiders, Donny & Marie, the boys are finally rescued by Wanda. Mr. Turner makes things up to Timmy by allowing him to watch the ending of the movie. Before they go to bed, however, Mrs. Turner appears in her night make-up scaring Mr. Turner so much that he hides behind the couch and he says that he powdered his nose in his pants again.
Note: The main character of I Was a Teenage Blowfly appears to be Dash Baxter from another Butch Hartman TV show, Danny Phantom.