Phineas and Ferb build a shrinking submarine so they can retrieve Isabella's sash (which was eaten by her chihuahua, Pinky). Unfortunately, they end up inside ''Candace'' by mistake.

Meanwhile, Dr. Doofenshmirtz builds a device that will destroy people who can't make up their mind.
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!!Tropes
* AnuscapePlan: During the song "Hemoglobin Highway", one of the lyrics say "Slow down before we reach the colon", implying that the brothers do ''not'' wanna go there.
* AwLookTheyReallyDoLoveEachOther: Perry's concern that Phineas and Ferb could be targeted by Doofenshmirtz's invention is the first sign that he sees them as more than just a cover identity.
* BeCarefulWhatYouWishFor: Doofenshmirtz, in an attempt to add some variety to his and Perry's routine, shifts around the steps of their daily plot, specifically this time telling Perry his scheme first. When Perry learns just how destructive his current scheme is [[note]]Specifically, that it has the potential to kill Phineas and Ferb[[/note]], Perry immediately immobilizes him, which Doof [[LampshadeHanging lampshades]].
* BreakingOldTrends: In-Universe. Doofenshmirtz tells Perry his scheme first rather than trap him.
* CrazyPrepared: Doofenshmirtz did actually prepare a back-up trap for Perry should he be immobilized early on and had a hairpin ready to pick handcuff locks. A hidden flat-surface magnet belt sticks Perry to the wall when he opens a bag, allowing Doof to free himself. He states he had another belt in another bag as a precaution, but this proves to be his undoing.
* EarlyInstallmentWeirdness: This is the ''only'' episode where Doofenshmirtz attempts to perform such a legitimately evil action as ''mass murder'' [[DisproportionateRetribution simply because of indecisiveness]] rather than his usual PokeThePoodle-type schemes. Anytime another villain shows up later on that tries something like this, Doof is [[EvenEvilHasStandards always shown to be against them]].
* FantasticVoyagePlot: The whole episode involves Phineas and Ferb originally shrinking themselves to enter Pinky's body, but end up in Candace's body by accident.
* HereWeGoAgain: Subverted. It looks like Candace is going to eat the re-miniaturized submarine again with Doofenshmirtz inside this time, but then Pinky eats the sandwich instead.
* IdiotBall:
** Yeah, Candace, let's eat the mysterious cheese sandwich on the plate on the floor of the empty garage. That's a ''[[SarcasmMode great]]'' idea! (Plus, aren't you supposed to be lactose intolerant?)
** Doofenshmirtz was smart enough to prepare a back-up trap for Perry in two separate bags. After Perry is immobilized by one, Doof pats himself on the back, but he's dumb enough to open the second bag just to prove how far he thought this ahead. It takes him a moment for him to realize how badly he screwed up.
* {{Irony}}: Doofenshmirtz says he's wondering if the Make-Up-Your-Mind-inator has the right name. He admits that being unable to make up his mind on this is rather funny.
* LampshadeHanging: Doofenshmirtz and later Monogram describe the usual routine in Perry plots.
-->'''Doofenshmirtz:''' Perry the Platypus! I--y'know, we always do the same thing. I have a scheme, you try to stop me, I trap you, I tell you my scheme, you escape, then we fight, and I'm defeated. Let's mix it up a little.
* NonIndicativeName: The Make-Up-Your-Mind-Inator's name implies that it would force people to make decisions quickly. It's actually a sort of DeathRay that Doofenshmirtz plans to use to '''destroy''' anyone who can't make up their mind.
* OOCIsSeriousBusiness: When Perry is told by Doofenshmirtz what his Make-Up-Your-Mind-inator does (kill all who cannot make up their mind (which includes Phineas and Ferb)), Perry immediately takes Doof down, rather than doing the pair's normal routine.
* ReactiveContinuousScream: The rare joyous variation occurs in the beginning of the episode, where Candace, Stacy, and Jenny exchange excited shrieks on-phone due to Candace's soon-to-be-date with Jeremy. Their constant shrieking annoys Phineas and Ferb, so they decide to figure out what they want to invent outside instead.
* ShrinkRay: One of the things Phineas and Ferb had to choose between to build during the episode. In the end, it was combined into one idea with the submarine.
* {{Squick}}: In-universe. When Phineas finds out that they were inside Candace and during her date with Jeremy (meaning they were also on a date with Jeremy), Ferb said both times that it was creepy on so many levels.
* TheStinger: In the sub inside Pinky, Doofenshmirtz realizes he's in a dog and vows to [[PokeThePoodle pee wherever he wants in the Tri-State Area]]. An EvilLaugh is then interrupted by a depressed Doof asking, "Oh, what am I doing with my life?"
* ThisIsWrongOnSoManyLevels: See above.
* WhatHappenedToTheMouse: It's not clear what happened to the Make-Up-Your-Mind-Inator after Doofenshmirtz was trapped inside the submarine.