To get the perfect birthday gift for Trixie, Timmy must learn how a girl thinks. What better way than to become a girl for the day? Naturally, both AJ and Chester fall in love with "Timantha". Cosmo and Wanda are useless, as they have switched genders in this wish mix up as well. Timantha must break his best friends’ hearts and still have time to get the perfect gift in time for Trixie's party.
At Dimmsdale Elementary School, it's recess and Elmer, Chester McBadbat, and A.J. are eating perfectly in sync while Timmy intently watches a small portable television. Elmer looks over at Trixie Tang and Veronica and asks what they're talking about. After hearing the two popular girls talk in a few "blah, blah, blahs", A.J. simply responds he doesn't speak girl. Timmy begins shouting at a character on screen, warning him that the girl will betray her again.
Elmer asks him if he's watching a "girly soap opera", to which Timmy replies that there is plenty of action on the show as well, which is immediately revealed to be a blatant lie, with the show's title, The Kissy Kissy Goo Goo Romance Hour. Timmy only weakly remarks that he likes the writing.
Trixie then begins asking the playground something, which the boys only hear as "blah blah" until A.J. translates with a "Geek-To-Girl Translator". She's asking who didn't get invited to her birthday party. The boys raise their hands. Trixie responds by saying that's "everybody who's anybody". Timmy, however, goes up to Trixie and says something...which Trixie hears as "blah blah blah" except for three words: really great gift. This has the intended effect. Trixie gives Timmy and his friends invitations, telling him not to forget the gift. Once she leaves, he tells the others that he plans to get Trixie a gift so she never forgets his name again. Kissy Kissy Goo Goo comes on again, prompting Timmy to run off.
Later, back in his room, Timmy goes through his things, looking for a gift while Cosmo and Wanda watch. After a stink bomb and a dead frog fail, Wanda tells him he needs to think like a girl, which Cosmo follows up by suggesting that if Timmy was a girl he could think like one. Both boys laugh uproariously at this suggestion, with Timmy saying, "Yeah, right, like I'd ever wish I was a girl!" This angers Wanda, who promptly zaps him with her wand, doing just that. A screen of flowers appears with a butterfly passing by revealing Timmy being turned into a girl. Cosmo clenches his teeth. Timantha then pulls and notices her ponytail and then her dress. Shocked by what she sees she runs over to a closet mirror reaching out to make sure the reflection was hers and then makes a loud girlish scream. Timantha then says to Wanda, "What did you do!? I didn't wish for this!" she models hearing a fox whistle than goes back to her normal stance. Wanda, polishing her nails with her wand, says, "Well... You said I, wish, and girl." Timantha responds saying, "Yeah well you know what I think" Cosmo quickly says, "Well who cares what you think? You're a girl now!" laughing very hard and causing Wanda to smile.
Timantha tries to think, a thought cloud comes behind her revealing girlish stuff like Unicorns, ballerinas, rainbows, bunnies and the color pink. A sudden realization dons on her, and Timantha says, "Hey, now that I think like a girl, I can totally use my girl brain to think up the perfect gift for Trixie." Cosmo agrees sarcastically saying, "Wow, that's a great idea little girly girly!" twirling while doing so. Wanda opens the door for Timantha and says, "Can I get the door for you Ma'am?" and at this Cosmo and Wanda laugh. Timantha, with a glare, says, "No, I'm going to the mall to find something Trixie would like, but just so you two can have a laugh at while I'm gone, I wish you were a woman and you were a man," saying it to Cosmo and Wanda respectively. They protest "What?", but Timantha reminds them that as her fairy godparents they must grant the wish, so each fairy reluctantly turns each other into the other gender being cloaked in a cloud with the male and female signs. Timantha congrats them and says "Congratulations you are now Cosma and Wando." Cosma responds, saying, "Oh my gosh, how do I suddenly know which boy bands rock out loud?" and Wando says, "I don't know why am I tempted to scratch myself and not care who sees?" Timantha says goodbye to her gender-swapped godparents and leaves.
The camera quickly changes to the mall. Timantha appears walking in a ballerina walk. She starts to say to herself "think like a girl" saying it over twice. She then notices a comic store asking herself, "Do I still like comics?" She then thinks it is okay because it included "muscular guys in spandex fighting crime" with a quick "cool" she heads over to it. As Chester and A.J. watch from a distance, Chester is questioning why "a girl" is entering a comic shop. A.J. is eluded as well, but decides she is so cute he will stare at her until she is so uncomfortable she has to talk to him. He falls lovestruck as Chester screams as if A.J. is dying and the scene shifts back to Timantha.
In the store, Timantha finds the last copy of Skull Squisher and reaches for it, but another kid grips it as well. The two fight, with Timantha winning, but in the process the other kid's hat falls off, revealing her to be Trixie, much to the other kid's shock. The popular girl begs Timantha to keep her visit to the comic shop quiet, out of fear everyone will think that she's weird. Trixie declares her, her new best friend before ripping up a picture of Veronica (causing the real Veronica, elsewhere in the mall, to scream in pain). The popular girl then asks her new friend's name, to which she responds with a weak Timantha. As Trixie leaves, she tells her new best friend to meet her at the salon later to hang. As Timantha leaves, she runs into Chester and A.J., A.J. offers her a dead frog, to which Chester panics and tackles him. The frog flies out of A.J.'s hands and splats at Timantha, drenching her with some water. Timantha notes even as a girl that is still gross.
Back at the house, Wando becomes impatient since they were supposed to meet Timmy an hour ago. While he sits watching television and eating a hot dog, Cosma obsesses over what to wear. Wando tells her to just pick something. Cosma goes into the bathroom, only for a splash and her yelling, "Would it kill you to leave the lid down?" Wando only gives a snarly comment in return. She then drinks some soda before burping.
At the mall salon, Timantha and Trixie are enjoying a spa date. Timantha says "This feels great!" Trixie agrees, saying, "It's like their scrubbing all your troubles away." They are having shampoo in their hair which quickly changes into having their nails done Timantha says "and they're showing my favorite soap opera, this place rules" while a TV shows the title Kissy Kissy Goo Goo. Trixie admits that if she met a boy who admitted he liked Kissy Kissy Goo Goo, she would go out with him even if he wore a pink hat. Timantha says, "Really? But he would never come in here." Trixie responds saying, "Well they should, if boys did more girl stuff then girls could do more boy stuff," reading a book labelled PONIES revealing it to be Timantha's Skull Squisher. Realizing Trixie is right, Timantha pulls her out of the salon and into an arcade, which shocks the boys there. A.J. tries once again to hit on Timantha by offering her a dead frog. Timantha finds that "gross" but Trixie finds it to be "cool". Chester panics and pulls A.J. with him into the bathroom. He then nails up the bathroom door to prevent the girls from getting in. He then explains the difference between boys and girls to A.J. and then states that is why both genders have different bathrooms. Chester is then interrupted when Veronica walks out of one of the bathroom stalls, sees the boys, and asks if she's in the wrong room. Chester screams that nothing makes sense anymore and begins tearing the boards off the door. A.J. then offers Veronica a dead frog.
As Trixie leaves, she tells Timantha that she's given her what she's always wanted, a friend that would understand her. She then remarks "If you were a boy, I would totally date you," before driving off in her limo, saying that she'll see her at the party.
That night, outside Trixie's mansion, Timantha wishes herself back into a boy before entering (and interpreting Cosma's request to wish her and Wando back to normal as "blah blah blah". AJ and Chester give her their gifts (firewood). She then asks her bodyguard if Timantha has made it, which she hasn't. Timmy then enters, giving Trixie a walkman. At first she hears him talking blah, blah, blah until she uses the Girl-To-Geek Translator. Now hearing him clearly, Trixie hears Timmy admit he likes girl things. She finds Timmy's confession so sweet, but under the pressure of being judged, she remarks that's why she has girl friends and throws him out, but not before getting his name right while giving him a friendly good bye.
Outside the mansion, Timmy is satisfied that Trixie remembers his name, and realized it happened because he saw Trixie as a person with varied interests rather than just a girl. He wishes Cosma and Wando back to the way they were born, and they revert to normal...with Cosmo getting a long, ugly nose that he was born with. They then try to figure out what to do now since Timmy's parents are not expecting him home yet. They end up back at the salon, where every woman there wants Timmy to date her daughter due to his sensitivity. The Kissy Kissy Goo Goo show then states on television that Roderick fell down an elevator shaft (in reality he quitted the show). Timmy then asks the customers who wants to watch wrestling, and all of them want to watch it. Timmy then changes the channel which shows Roderick having his evil twin Roderick in a toe hold. Timmy and the customers all say, "Roderick, no!" as the episode ends.