After watching an old sci-fi movie, Phineas and Ferb are inspired to create a teleportation device which causes Candace and Perry to switch bodies. Meanwhile Dr. Doofenshmirtz has a grudge against a company mascot.
Phineas and Ferb are watching an old sci-fi movie about a man who created a teleport device. Phineas decides to create their very own teleport device. Linda crawls in and asks the boys if they have seen her earrings, and Phineas tells her that they'll keep an eye out for it. Phineas and Ferb run out to the garage and start building their teleport device.
Candace wakes up and sees Perry sleeping on her bed. She picks Perry up and goes outside to tell Phineas and Ferb to keep Perry out of her room. On her way, she stops by the kitchen to tell her mom that Jeremy is coming over and to try not to embarrass her again. Phineas and Ferb are about to take a picture of the completed teleportation devices when Candace jumps in front of the shot asking them how many times she's asked them to keep Perry out of her room. She then takes notice of the teleports, and as she's going inside to tell Linda, Candace trips on a wire in the garage and falls into the teleport device with Perry. This causes them to switch brains, which worries Candace because Jeremy will be there any second. Phineas tells Candace that they'll switch them back in no time, but when they turn around, Perry in Candace's body has gone off to his lair, a tight squeeze, considering that the door he's using is platypus sized. Phineas, Ferb, and Candace in Perry's body soon recruit Isabella for help.
When Perry arrives at his lair, we can hear Major Monogram yelling at Carl. When Monogram sees Candace's body, he thinks that there has been a breech in security. Carl gets excited because he thought that it was a real teenage girl, but after donning the fedora, Monogram and Carl assume that Perry has disguised himself as a teenage girl, causing Carl to walk offscreen, complaining that he thought it was a real girl. Perry takes a look at himself and realizes that he and Candace must have switched brains. Perry then receives his mission. Apparently Slushy the Clown statues have been disappearing all over the tri-state area, and lunchtime just isn't fun anymore without those clowns. Perry then sets off, but on his way, he has an encounter with Stacy and her mom. "Lost Platypus, looks like a girl!"Meanwhile, Phineas and Ferb decide to put up fliers of Candace holding Perry to help them find Perry in Candace's body. Candace is devastated when she sees the picture that the boys decided to use, but it was the only picture they had. Phineas then complements Isabella on her blimp with an enlarged version of the photo.
Over at Doofenshmirtz Evil Incorporated, we can see Doofenshmirtz ripping the cassettes out of the Slushy the Clown statues. Perry in Candace's body then bursts through a wall, causing Doofenshmirz to be confused. He then traps Perry, apologizing for the tight fit, because he didn't know that Perry was going to be a teenage girl, then explained his evil scheme. He was sick and tired of hearing the Slushy the Clown jingle, so now he's going to replace it with his own evil jingle, but he's having a hard time finding a rhyme for "evil".
After the boys are finished posting the fliers, a police officer comes over and asks about the fliers. Phineas explains that they're looking for a platypus that looks like a girl. Phineas also asks him for help finding Linda's earrings, which are still missing. The fliers are attracting people from all over town, who are wondering how they're supposed to know it's a girl.
Inside the Flynn-Fletcher Household, Candace is trying to tell her mom that she's been turned into a platypus when Jeremy arrives. She asks Linda to tell him that she's getting ready. Linda answers the door and invites Jeremy in, then goes off into the kitchen to make him a snack. Jeremy then looks under the couch and sees Candace in Perry's body. He pulls her up next to him and starts scratching her head, causing Candace to giggle. She quickly corrects herself, doing an imitation of Perry's chirping sound. Linda then walks in with some sandwiches, then turns on some embarrassing baby movies of Candace. Candace runs into the kitchen to remind Linda about the fifth grade graduation. Linda still doesn't get it, so Candace asks her to turn off the home videos.
Perry finds a bobby pin in Candace's hair and uses it to escape the cage he's in. Just as Doofenshmirz is complaining about how everything rhymes with "fun", Perry knocks him over. He then attempts to attack Doofenshmirz with his tail, but since he no longer has a tail, the attack fails. Doofensmirz points this out, saying "Now you are no match for me." Then he stops himself, saying he can't hit a girl, when Perry attacks him with a purse. Doofensmhmirz then attempts to escape, but doesn't go anywhere, leaving him wondering what happened.
Later, we see Jeremy feeding Candace platypus food, and Linda is looking for her earrings again. Linda thanks Jeremy for feeding Perry, and Jeremy asks what's in it. Linda says that it's mostly worms and insect larvae, causing Candace to barf all over the carpets. Linda puts her outside, complaining about having to get the carpet cleaned again. Linda then realizes that she could have lost her earrings when she was doing the laundry and asked Jeremy to help. Candace wanted to be let back in, and as she was knocking on the glass, she realized she was sweating milk.
He then explains that he can't fight Perry when he's dressed as a girl, because "it's just so... como ce dice ("how do you say" in Spanish)... awkward". Doofenshmirz then exclaims "Look! Shoes on sale!" causing Perry in Candace's body to look, letting Doofenshmirz escape. But Perry quickly pulled out Candace's iPod and tied up Doofenshmirz. Perry then returned all of the Slushy the Clown statues to their restaurants before returning home himself.
Perry arrives back just as Candace is complaining to Phineas and Ferb that Linda is ruining her love life. They immediately rush to their garage, where Candace holds Perry, and after hearing Candace complain that Perry was a lot heavier now, Phineas pushes a button causing Perry and Candace to switch back into their own bodies. Candace immediately tries to bust Phineas and Ferb and runs inside the house. Dr. Doofenshmirtz is trying to get out of the iPod, but since he cannot move, he has to navigate himself by jumping. He loses his balance and hits his Shrink-inator, stating that he should have unplugged it before turning it into a planter. The Shrink-inator hits the teleport device and shrinks it so that it looks like small hoop earrings. Linda arrives in the garage and believes that Phineas and Ferb built her earrings and thanked them.
|The image gallery for this episode may be viewed here.|
 End Credits
 Running Gags
 The "Too Young" Line
|When Candace got her body back.You might consider bathing first.|
 Whatcha doin'?
|Isabella: Whatcha doin'?Phineas: Hey Isabella, we built a teleportation device and accidentally switch Candace and Perry's brains.
Isabella: Really? That's so cool.
 Perry's entrance to his lair
Perry, in Candace's body, squeezes into the hidden door on the side of the house.
 Memorable Quotes
|Candace: Am I sweating milk?! Being a platypus is so gross!|
|Carl: Man! I thought it was a real girl.|
|Dr. Doofenshmirtz: A teenage girl?Dons the fedora
Dr. Doofenshmirtz: Perry the Teenage Girl!
|Candace: How did you get so big? And why is my hand green?Candace looks in the mirror and screams
Candace: I'm an ugly, smelly platypus!
|Doofenshmirtz: Sorry about the tight fit, but you know, if I had known you were going to disguise yourself as a teenage girl, I would have set a bigger trap.|
|Stacy: (Thinking Perry is Candace) Hey, Candace! Awesome scooter! Hey, do you wanna eat at Slushy Burger with me and my mom?Perry: (tips his hat off to Stacy and gives a thumbs up at a green light before driving away)
Stacy: Was that a yes or a no? Dr. Hirano: I don't know. But I like the hat.
Stacy: Yeah, it's a good look for her.
|Doofenshmirtz: Keevil...Beevil...Heevil...Steevil...Perry the Teenage Girl unlocks the trip with a berette...
Doofenshmirtz: As soon as I get this done, lunchtime will never again be fun. OH GREAT! THERE'S A RHYME! THERE'S A RHYME! YEAH, I'M SURE EVERYTHING RHYMES WITH "FUN"!
 Background Information
- 36th episode produced. 37th episode aired, first broadcast on August 10, 2008.
- Slushy the Clown looks like a combination of Ronald McDonald and Big Boy
- The "I know what we're gonna do today!" line is for the first time said by someone else, by the mad scientist on the TV.
- This episode was the "never before seen" episode that could be unlocked on the Disney Channel website if enough questions were answered correctly in their first Spot the Diff contest.
- On the credits, it is shown that when in Candace's body, Perry's parachute changed from green & white to a picture of Jeremy with hearts around him.
- Doofenshmirtz refers to his old "Shrink-inator", but the invention he activated actually most resembled the ray to destroy the Moon in the episode Oil on Candace; his invention to shrink things was called Shrinkspheria. He says he should have unplugged it, since he turned it into a planter, yet forgets as we see in Let's Take a Quiz.
- During the time that Candace was in Perry's body, her eyes had eyelashes. Perry normally does not have eyelashes.
- This is the fifth episode with a song in the end credits that wasn't featured in the episode.
- This episode in Spain is call "Este Ornitorrinco me hace parecer gorda?" (that translated is "Does this platypus make me look fat?"
- This is one of the episodes where Doofenshmirtz uses a different language saying, "Wait, wait, I can't fight you when you're dressed as a girl! It's so, ¿cómo se dice?... awkward..." Translated from Spanish is saying: "as stated" or "how is it said"
- Pinocchio: Carl utters "I though it was a real girl", a reference to the line "real boy" from the famous story.
- Title: A notorious "trick question", "Does this dress make me look fat?"
- Ronald McDonald and Big Boy: The look of Slushy the Clown is a combination of Ronald McDonald and Big Boy, the famous clown mascots for their respective restaurants.
- The Simpsons: The Slushy the Clown also looks like Krusty the Clown from The Simpsons.
- Roadrunner: When Doofenshmirtz attempts to run away in a cloud of smoke and fails, it resembled how Roadrunner makes its getaways.
- Wrongfully Accused: Ferb gets tangled in the wires much like the main character of this movie.
- Perry the Teenage Girl: A possible reference to Kim Possible.
- Vincent Martella as Phineas
- Ashley Tisdale as Candace
- Thomas Sangster as Ferb
- Caroline Rhea as Mom
- Alyson Stoner as Isabella
- Dan Povenmire as Dr. Doofenshmirtz
- Jeff "Swampy" Marsh as Major Monogram
- Dee Bradley Baker as Perry
- Mitchel Musso as Jeremy
- Maulik Pancholy as Baljeet
- Kelly Hu as Stacy
- Bobby Gaylor as Buford
- Tyler Mann as Carl
- Ming Wen as Dr. Hirano
- Additional voices: Carlos Alazraqui, Dee Bradley Baker, Jennifer Hughes, Maulik Pancholy, Rob Paulsen
- ♦ designates a character that did not appear in this episode
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