Does This Duckbill Make Me Look Fat?

After watching an old sci-fi movie, Eric and Peter are inspired to create a teleportation device which causes Gina and Perry to switch bodies. Meanwhile Dr. Sabatino has a grudge against a company mascot.

Summary
Eric and Peter are watching an old sci-fi movie about a man who created a teleport device. One of the characters threatens to remove the tongue of another character, so when Eric says what they're going to do today, Peter immediately covers his mouth. Eric quickly reassures him that he was talking about creating their very own teleport device, not removing anybody's tongue.

Just then, Hunter crawls in and asks the boys if they have seen her earrings. Eric asks her which ones they were (sparkly, blue sapphires or the hoops) and Hunter says she lost her hoops, which Eric instantly interprets to mean that she's wearing "the purple dress". Eric tells her that they'll keep an eye out for it and once she has crawled away, the boys run out to the garage and start building their teleport device.

Peter does most of the building, while Eric handles ordering the supplies and materials, and soon the teleporting devices are complete. Eric wipes the sweat off of his brow and asks where Perry is.

Gina wakes up and sees Perry sleeping on her bed. She strongly rejects, saying, "Eww, you're not supposed to be here!", picks Perry up, and storms out to the garage to tell Eric and Peter to keep Perry out of her room.

On her way, she stops by the kitchen to tell her mom that Joshua is coming over and to try not to embarrass her again. Hunter asks what she means, and Gina reminds her of an incident at her senior college graduation that involved something inflatable.

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 teleporters, but just as she's racing inside to tell Linda, Candace trips on a wire in the garage and falls into the one of the teleporting devices with Perry.

She emerges out of the other one sprawled on the garage floor. Phineas asks her if she's okay, but it turns out that this caused them to switch brains, which distresses Candace because Jeremy will be there any second. Candace sees her platypus self in the mirror and shrieks in horror. Phineas tells Candace not to worry, because they can switch them back in no time, but when they turn around, Perry in Candace's body has gone off to his lair.

Perry is trying to go off to his lair, anyway. He uses the side-of-the-house entrance, which is a tight squeeze, considering that the door he's using is platypus sized. The door has only just managed to close when Phineas, Ferb, and Candace in Perry's body come into the backyard. Isabella comes in too, and once they've explained the situation to her, Phineas, Ferb, and Candace in Perry's body recruit Isabella for help.

When Perry manages to squeeze through the pipes and arrive at his lair, we can hear Major Monogram yelling at Carl. When Major Monogram sees Candace's body, he thinks that there has been a breach in security, because instead of Agent P he sees Candace. Carl gets excited because he thinks it's a real teenage girl, but after donning the fedora, Major 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 good 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. Major Monogram and Carl are especially heartbroken about the loss of the clowns, for they eat lunch there every day.

Perry then sets off on a little scooter, but on his way, he has an encounter with Stacy and her mom. They stop at the same stoplight as Perry, and Stacy tries to call out to him. Perry in Candace's body somehow avoids talking to them, even as Stacy compliments his scooter and invites him to lunch at the Slushy Burger, thinking it's Candace. Then the light turns green and he zooms off with relief, leaving Stacy and her mom wondering about his/her behavior and complimenting his/her hat.