[[quoteright:350:https://static.tvtropes.org/pmwiki/pub/images/img_4889.png]]
[[caption-width-right:350:"Oh, hi, Vanessa!"]]
->''Well, I'll be honest, I don't really understand\\
But I fell down this hill, now I got glue on my hands\\
And I got records on my fingers!'' (Whaaaaat?) ''Records on my fingers!\\
I got records on my fingers, and I just can't stop.'' (Don't stop!)\\\
''I can't stop, I got a platypus controlling me!'' (Whaaaaat?)\\
''I got a platypus controlling me!''
-->-- '''Dr. Doofenshmirtz''', "There's a Platypus Controlling Me"

Doofenshmirtz makes a De-Volitionator which causes Perry to do whatever he wants, which distracts him from picking up Vanessa at a teen party in the dump.

Meanwhile, Phineas, Ferb, and their friends (except Buford) are sick, so they find a way to make it fun by playing video games.
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!!"Brain Drain" contains examples of:
* AnswerCut: When Baljeet says, "It's not like Ferb is an internationally ranked video game wizard or anything", the camera cuts to Ferb sitting beside a trophy with "World Champion" written on it.
* ArtisticLicenseCars: The Evil Jingle introduces Charlene's car as "Doofenshmirtz's ex-wife's sports sedan", but it appears to be a 1973 Chevy Camaro.
* BlandNameProduct: Monograms briefs Perry in by stating that Doofenshmirtz has been seen buying from Brain Bath and Beyond, "the area's largest Big Box Mind Control Warehouse store".
* BossOnlyLevel: When Doofenshmirtz plugs the De-Volitionator into a speaker system to recharge it, it sends a power surge down the electricity line that shorts out the kids' gaming system. This means that the kids are able to control the De-Volitionator using their gaming controls. They assume that they're playing the boss level of the game (complete with super-realistic graphics).
* BrickJoke: When Doofenshmirtz arrives at the party, he tells Perry the Platypus that they should have hosted the party in a nicer place like the Caribou Lodge. Later, when Doofenshmirtz gets his hands stuck to the turntable, one of the punks says, "I was at the Caribou Lodge once, and it was just like this," implying that Doofenshmirtz has done this before.
* BringIt: Once the kids gain control over Perry, Phineas makes a point of having him assume a martial arts stance, complete with "Bring It" taunt.
* CatchphraseInterruptus: Phineas starts to say, "I know what we're gonna do today!", but coughs midway through.
* ContinuityNod:
** When introducing the De-Volitionator, Doof mentions it's not to be confused from the De-Evolutionator from "[[Recap/PhineasAndFerbHawaiianVacation a few schemes ago]]".
--->'''Doofenshmirtz''': You see, volition is our ability to choose what we are going to do. Whereas evolution... eh, that's got something to do with monkeys, I think.
** Also:
--->'''Doofenshmirtz:''' You remember Vanessa, right? You know... [[Recap/PhineasAndFerbDudeWereGettingTheBandBackTogether You blew me up at her birthday party.]]
* ADayInTheLimelight: For Doofenshmirtz.
* DemotedToExtra: Candace only appears in the beginning of the episode.
* GrossGumGag: One of the things Dr. Doofenshmirtz makes Perry the Platypus do with his mind-controlling De-Volitioninator is pick up chewed gum from the sidewalk. Doofenshmirtz stops Perry before the latter actually puts it in his mouth, but then he makes the platypus go through it a second time.
* IAmNotWeasel: When the gang controls Perry, Buford suggests they just fight "that old pharmacist", referring to Doof.
* ILetYouWin: Subverted. When Baljeet asks if Ferb let him win to save his ego, Ferb looks up while reading a book and a drinking bird is pecking at his controller and says, "Oh, I'd never do that."
* ItsTheBestWhateverEver: The boys plan to make this the "best sick day ever".
* LettingTheAirOutOfTheBand: When Phineas starts to say his catchphrase, the EtherealChoir starts up, then he coughs, causing the music to wind down to a stop.
* MirrorMatch: During the gang's video game fight, after Buford fights Isabella, and Baljeet fights Ferb, next up is Phineas... against Phineas. The two Phineas attack each other and both got knocked out, and the game announcer exclaims "PHINEAS WINS!"
-->'''Phineas:''' I'm kicking my own butt!
* MistakenForProfound: The kids at the rave think "There's a Platypus Controlling Me" is a metaphor for "whatever's keeping you down."
* NoodleIncident: In a flashback, Doof picks Vanessa up dressed as a baby on a tricycle pulling a red wagon. Doof simply says to her, "[[PreEmptiveShutUp Don't ask]]."
* RecordNeedleScratch: One occurs, appropriately enough, when Dr. Doofenshmirtz interrupts the rave and gets his hands glued to the turntable.
* ShoppingCartAntics: Doofenshmirtz is able to force Perry the Platypus to steal a shopping cart and use it to push Doofenshmirtz to the rave where Vanessa is at. Someone even complements him for it:
-->'''Bystander:''' Sweet ride, dude.\\
'''Doofenshmirtz:''' Thank you. Platypus powered!
* SickEpisode: On the kids' end. With the exception of Buford who is just lazy. Although, by the end, their fevers break, but Buford ends up getting sick.
* SmokingBarrelBlowout: Ferb does this to his controller after the gang defeat "the old pharmacist".
* StopHittingYourself: Doofenshmirtz does this to Perry with the De-Volitionator.
** Later, Buford does this after the kids' video game system starts controlling the De-Volitionator.
* ThrowTheDogABone: Perry uses the De-Volitionator to help Dr. Doofenshmirtz bust a rap and impress Vanessa's friends.
* VerbalBackspace:
-->'''Vanessa:''' Dad?\\
'''Johnny:''' That's your dad?\\
'''Vanessa:''' No!
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-->''"[[TheStinger Whuuuuuut???]]"''