Timmy borrows Mark Chang's "iFake" disguising device in order to attend Trixie Tang's costume party, unfortunately his lack of knowledge about Yugopotamian technology puts him in a tight spot when he accidentally puts the device in shuffle mode, turning him into different monsters!
Timmy is sleeping outside of his house in front of the mail box, eagerly awaiting the arrival of an invitation to Trixie Tang's annual costume party. However, there is no invitation when the mail comes, although the mailman himself was invited, as were Cosmo, Wanda, and Poof. Timmy asks how his fairies can get invited when Trixie doesn't even know they exist, and Cosmo reminds him that Trixie doesn't know Timmy exists either. Back inside his bedroom, Timmy complains about the situation and learns that even his own parents were invited as well. His parents were costuming as "the perfect woman", in other words they switched identities (Mr. Turner was his wife and Mrs. Turner was her husband). Timmy cannot use his fairies either to wish himself a good costume, because the costume party is technically a competition, and Da Rules prevent magic from being used to win competitions. So, Timmy decides to use Yugopotamian technology instead.
Timmy travels to the Dimmsdale Junkyard where Mark Chang's spaceship is "hidden". There, he learns that Mark has also been invited to the party, and is going to use his "iFake" to change into the perfect costume for the party. Timmy tells Mark that he can just go as his normal alien form and it will be a great costume. Mark Chang agrees and goes to the party "in the buff", leaving Timmy to borrow the Fake-i-fier. Timmy uses the device to change himself in "Rex Perfection", a handsome prince with a smile so bright it literally brings plants back to life. That night at Trixie's mansion she has her party, and Cosmo, Wanda, and Poof are relieved that they can appear in public as themselves for once, since no one will notice them at a costume party, except of course for Mr. Crocker who is also (crudely) dressed as a fairy. Mr. and Mrs. Turner arrive and immediately get kicked out when Mrs. Turner mistakes Trixie's princess costume for a "bratty, stuck up, snotty little rich girl" costume. Trixie welcomes her guests to the party, and then criticizes everyone's costume, demanding someone show her a more impressive costume. Timmy arrives as Rex Perfection and everyone, including Trixie, is immediately amazed by his costume. Trixie quickly jumps to his side. All is going well for Timmy, until he bumps into another party goer which puts the Fake-I-fier into shuffle.
The Fake-i-fier begins to change Timmy into random monsters and disgusting creatures, each time causing Trixie to hit him over the head and then run away. Timmy tries to find Mark for help fixing the device, but only ends up tearing up a costume that looked similar to Mark worn by Chet Ubetcha, and accidentally releasing a green hose tied to a railing Timmy's parents were trying to use to scale the mansion to get back into the party. Timmy changes back into Rex Perfection and he and Trixie begin to dance, but in the middle of the dance while Timmy sends Trixie twirling, he changes into a variety of dangerous objects (such as a chain saw) whenever Trixie starts to twirl back toward his arms. Meanwhile, Mr. Crocker is bothering Timmy's fairies with his lame magic tricks, so they use real magic to zap Crocker with a lightning bolt. Meanwhile, Timmy changes into a Mark Chang look alike, causing Trixie to declare him "Mr. Crocker hideous" and run off again. Timmy finally finds Mark, who was hanging with a "harem" of girls (actually guys wearing full-body girl costumes). Mark is angry at Timmy for taking his Fake-I-Fier without permission, and tells him that he has it in shuffle mode. The device then changes Timmy into Rex Perfection and back to himself, to the cheers and boos of the other party goers. Finally, the Fake-i-fier turns Timmy back to normal, he is thrown out of the party.
Outside of Trixie's house, Timmy muddles over the situation he got himself in, Trixie was more interested in Rex Perfection than the actual Timmy Turner. Just then, Trixie appears outside and tells Timmy she did invite him. Cosmo simply misread the mail and the invitation was for Timmy, not his fairies. Timmy asks Trixie if she really did want him at her party, and she replies "Of course I did, you're dreamy!" and then unzips her suit to reveal it was really Tootie in disguise as Trixie. Tootie tells Timmy to pucker up as she tries to kiss him. He yells for his fairies to help. They use magic to set off a spring trap in front of Trixie's door, launching Tootie onto the street into trash cans. The real Trixie shows up and gives Cosmo, Wanda, and Poof a trophy full of shrimp for having the best costume. Timmy then asks the real Trixie if she really did want him at her party, she responds by pulling a lever, launching Timmy to the trash cans and got kissed by Tootie.