Phineas and Ferb are watching an old sci-fi film where a mad scientist has constructed a teleporter but his assistant realises that unless there's another teleporter at the desired destination he can't achieve his aim, whereupon the mad scientist threatens to cut his tongue out. This inspires Phineas and, after clarifying his idea with Ferb, they commence to construct their own working teleporters based on the layout shown in the film; a chat with their mum reveals that she's lost her hoop-shaped earrings. Once the construction is complete Phineas wonders where Candace has got to.
It turns out that Candace is still in bed, but is roused when she smells Perry, who is sleeping on her bed. Angry with this intrusion she decides to confront Phineas and Ferb with Perry in her arms. She checks with her mother as to the boys' whereabouts and then asks her not to do anything similar to an event during her 5th grade graduation where she was embarrassed in frontof Jeremy, who just happens to be popping round later in the day.
In the garage Phineas checks with Ferb to ensure that they can make a photographic recording of their latest scheme; unfortunately Candace gets in the way of the shot at the critical moment as she tells them off for letting Perry get into her room. Immediately afterwards she notices the teleports and, confident that she can surely bust them, doesn't watch her step and stumbles into the teleporter with Perry, coming out the opposite teleport. Phineas is pleased that Candace has proved that it works and decides to check how she feels after having her body teleported.
Candace is bewildered as Phineas and Ferb seem substantially bigger than before and becomes unnerved when she sees that her hand is green; reaching for a mirror she freaks out upon seeing that she is a platypus! As she rants and raves Phineas understands that, because Candace and Perry both went through the teleporter together, their brains must've been swapped. Candace confronts them for getting her into this mess but is then distracted by the fear of what Jeremy would say if he found her in the body of a platypus; Phineas is confident that all they need to do is repeat the process, unfortunately Candace's body is nowhere to be seen.
Perry, unaware that he's now a girl, has squeezed himself into one of his tunnels and is on his way to the lair. Just as everything returns to normal appearance Phineas, Ferb and Candace enter the scene in search of Perry. Isabella now pokes her head round the face and asks what's going on, whereupon Phineas fills her in on the situation and she responds in an impressed manner, which Candace doesn't think would happen if Isabella were in her shoes.
Meanwhile Perry has entered his lair and activated the video phone; Major Monogram reprimands Carl for lowering the seat upon which he sits for the debriefing as he returns it to the standard elevation, as he starts to address Perry he immediately believes that there's been a breech of security upon seeing Candace in Perry's seat. The news of there being a teenage girl in Perry's lair catches Carl's attention in an excitable way, until Perry's hat falls onto his/her head and Monogram immediately realises that it is Perry in there. Monogram's praise for his 'disguise' and Carl's reaction of disappointment confuses Perry and, getting a look at his front paws, becomes concerned and checks his reflection in the control panel; seeing Candace staring back he makes his trademark noise in the sense of surprise and alarm. While Monogram debriefs him about the disappearance of Slushy the Clown statues and both he and Carl sing the fast food jingle in question Perry is looking over his new anatomy; Monogram's immediate order to find the statues before lunch makes him forget his bodyswap and instead sets about the assigned mission. As he's on his way by scooter Stacy and her mother spot him, however as he's inside Candace's body Stacy believes it to be Candace and asks if she's like to come along for lunch; since Perry cannot communicate by speaking he just smiles, tips his hat and then gives a thumbs up before continuing along the road.
Phineas and Ferb have been making 'Lost Platypus' posters in the meantime while Candace worries over Jeremy's imminent arrival; however their posters get her even more worried, not so much for their blunt honesty (lost platypus, looks like a girl) but for their use of a photo of her looking really cross as it was the only one they had to hand. What really got through to her was that Isabella was manning a blimp upon which a huge version of the poster was blazened, leaving her to fret as Phineas and Ferb go about putting up the posters.
At Doofenshmirtz Evil Inc. Doofenshmirtz has been taking all the tapes out of the Slushy the Clown statues he's stolen when he's surprised to see a teenage girl breaking through his wall and is shocked when Perry puts his hat on, revealing his identity. Doofenshmirtz activates his platypus trap and apologises for the of the cage in ratio to Perry's current scale, even offering a larger cage had he known that Perry was going to be a teenage girl. Now Doofenshmirtz commences with his evil 'genius' monologue: With the Slushy the Clown jingle going around and around in his head for the past week he's decided to remove all recordings of the jingle and replace them with recordings of his own evil jingle, preferring to write a new one instead of including his existing jingle; the only problem is he can't find a rhyme for evil in spite of his best efforts (Keevil, beevil, feevil).
At this time Phineas and Ferb confirm their poster's claim with a policeman that they've lost their platypus, whereupon he radios in exactly what the poster says; while he's their Phineas also points out that his mum is missing her earrings, which the bobby inquires as to which earrings before accepting this task as well.
Back home Candace attempts to bust her brothers, seeing as if her mum sees her in Perry's body while using her original voice then there'd be undeniable evidence, unfortunately Linda is too busy searching for her earrings and the doorbell rings before Linda can turn her attention to Candace; the high possibility that it's Jeremy at the door makes Candace look for cover so that the bodyswap doesn't come to light with him. Linda answers the door to Jeremy and invites him to wait in the living room and offers to make some sandwiches; while she's out of the room Jeremy spots a platypus hiding under the sofa and naturally assumes it to be Perry. Upon placing the semi-aquatic mammal on the seat next to him he scratches its head and almost sweet-talks it; the response from Candace is initially a feminine cooing and giggling before catching herself and doing the closest she can to Perry's trademark noise.
In downtown Danville Phineas and Ferb are now placing their posters on billboards, blown up to fill the spaces; the public read them and are confused by its content as to whether or not the girl is missing a platypus or the platypus is the girl as Phineas and Ferb have said.
Back in the Flynn-Fletcher household Linda offers Jeremy some home movies to keep him occupied, unfortunately she picks some videos of Candace as a baby; seeing this as a threat to her relationship Candace walks on all fours out of the room and shouts back a warning before resuming her position next to Jeremy. Linda doesn't recognise the significance at first so an exasperated Candace walks out again and tells her to stop playing the videos before returning to her seat.
In Doofenshmirtz Evil Inc. Doofenshmirtz is still trying to find a rhyme for evil (meevil, steevil, pleevil, heevil) and declares that there will be no more fun in lunch when he's done and gripes over the fact that that rhymes; while he's been fussing over his jingle Perry's found that one of Candace's hairpins makes an excellent lock-pick and sets on Doofenshmirtz. The surprise attack throws Doofenshmirtz onto the floor but once the confrontation gets going Perry forgets that teenage girls don't have tails and Doofenshmirtz feels confident that Perry doesn't stand a chance now, until he realises that he doesn't have the forebear to lash out at a girl. Now Perry comes at him wielding a handbag like numbchucks; this new threat makes Doofenshmirtz decide to leg it, until he finds himself standing exactly where he was before.
In the Flynn-Fletcher household Jeremy is unwittingly feeding Candace platypus food, believing her to be Perry for obvious reasons; a comment from Linda regarding Candace's dedication to him in her room seriously embarrasses Candace and she does her best to hide her personality from Jeremy's view while Linda finally realises that she's just said something she shouldn't and instead thanks Jeremy for feeding Perry. Candace has been eating the platypus food quite well until Jeremy asks what the platypus food is comprised of and Linda tells him that it's mostly worms and insect larvae; this news immediately sets of a psychosomatic reaction as Candace finds herself incapable of vomiting all over the carpet, resulting in her expulsion from the house. Just then she hears her mother asking Jeremy to help her search for her earrings in Candace's laundry; Candace now finds herself desperate to get back in and she begins to perspire, much to her chagrin when she realises that it's milk.
Back in Doofenshmirtz Evil Inc. Doofenshmirtz is complaining that it's an unfair fight if Perry's dressed as a girl, making it awkward for him to counter-attack; just then Doofenshmirtz makes to point out a shoe sale, which somehow catches Perry's attention while Doofenshmirtz legs it, until Perry improvises a lasso-type capture with an MP3 and earphones and Doofenshmirtz curses once again as Perry makes off with the Slushy the Clown statues. His mission to return the statues brings praise from witnesses, who believe he's the girl missing a platypus and not a platypus who looks like a girl.
Outside the Flynn-Fletcher household Candace is griping to Phineas and Ferb over how their mum is making all sorts of embarrassments for her regarding Jeremy. Just then Perry returns playing his c 'mindless pet' routine and Candace demands that they return to their own bodies. In the garage Phineas checks that they're in the same position they were when they first went through the teleporter, which now means that Candace is trying to avoid giving Perry's body a hernia as she holds her own body up above her head. Just as the teleportation is complete Candace and Perry tumble out, back to their original orientation. Candace moves to spill the beans to their mum but Ferb suggests that she have a bath first.
In Doofenshmirtx Evil Inc. Doofenshmirtz is hopping around, trying to untangle the earphones that bind him, until he stumbles into his Shrinkinator, which is now a planter but hasn't been unplugged, and a beam fires from it across town.
Back in the Flynn-Fletcher household Candace does her best to get her mum to see what's in the garage as soon as possible; however just as she reaches the door the Shrinkinator's beam comes straight through and shrinks the teleports, making Linda believe that Phineas and Ferb had improvised replacement earrings. It's all in a day's work really.