Isabella's dog, Pinky, eats her Fireside Girls sash, so the boys build a mini-submarine to retrieve it, but they end up inside Candace instead. Perry is forced to take direct action when Dr. Doofenshmirtz's latest scheme specifically threatens Phineas and Ferb.
Phineas and Ferb can't make up their minds on whether to build a submarine or a shrink ray. Linda says that they have wonderful imaginations. Candace is eating her favorite cereal, when Jeremy calls. He asks if she would like to go for a walk later, causing her to spit her cereal and milk out. She accepts and then calls Stacy and Jenny screaming with excitement. Meanwhile, in the backyard, Phineas and Ferb are still trying to decide what to build. Isabella walks along with her dog, Pinky, to show them her Fireside Girls sash with all her accomplishment badges. However, when she does so, Pinky jumps up and eats it. Isabella gets upset because she needs it for the ceremony that afternoon. Ferb shows Phineas their blueprints and they decide to create a shrinking submarine to retrieve it from Pinky's stomach.
Back at the house, Phineas explains to Isabella that he and Ferb will shrink down into Pinky's dog food and when he swallows them they will fold up the sash to microscopic size and exit through his drool. Isabella says that he will eat anything but dog food, though he does like grilled cheese sandwiches. Phineas says that he and Ferb will build the submarine while she makes the sandwich. When she returns with it, she finds Phineas with the completed submarine. She places the sandwich on the ground; Phineas and Ferb activate the submarine and drop into it. However, when Isabella places Pinky on the ground, he runs off chasing a cat instead of eating the sandwich. After Isabella leaves to chase after him, Candace comes into the garage to tell the boys to stay out of her way as she goes on her date with Jeremy. She sees the sandwich on the ground and decides to eat it so she doesn't look like a pig on her date.
Meanwhile, Perry breaks into Doofenshmirtz Abandoned Vacuum Cleaner Factory and Dr. Doofenshmirtz suggests they "mix it up a little" this time. So Doofenshmirtz tells Agent P his plan before he captures him. He shows Agent P the "Make-up-your-mind-inator" he invented to destroy people who can't make up their mind. Then, Perry remembers that at breakfast, Phineas and Ferb said they couldn't make up their minds about what they were going to do that day. Enraged at this, Perry then cuffs Doofenshmirtz to a nearby pipe and calls Major Monogram to send backup to come get Doofenshmirtz.
Phineas and Ferb are sitting in the submarine in Candace's mouth waiting to be swallowed. When Candace hears the doorbell ring, she swallows the submarine and goes to meet Jeremy at the door. As they go down her throat Phineas comments that Pinky's neck is really long. When they get to her stomach and search for the sash, he notices tacos similar to the ones Linda made for dinner the night before and then Candace's favorite cereal. When he begins to realize what's going on, Phineas gets a call from Isabella, who is wondering where they disappeared to, inadvertently confirming his suspicion. Phineas hangs up and tells Ferb they are inside Candace instead. Ferb replies with "That's creepy on so many levels."
While they are waiting, Doofenshmirtz tells Perry that there is travel chess set in one of two nearby handbags. Sensing a trap Perry opens the other bag. When he opens it, there is a self-attaching belt that sticks itself to any flat surface which traps Perry. Dr. Doofenshmirtz frees himself and shows Perry that there was one in both bags so no matter which one he picked he'd be trapped and unintentionally traps himself.
Inside Candace the boys tap into her eyes and ears through her nerves to see what she sees. They see that she is on her date with Jeremy and realize that means they're on a date with him. Phineas calls Candace to tell her and she doesn't believe him. To prove it Ferb uses a feather duster to tickle her. Phineas sees some monkey bars and asks her if she still gets sick when she hangs upside down. He apologizes, but tells her they have to help Isabella as they tap into Candace's arms and legs. They walk her over to the bars and make her hang upside down and they begin to move out of her stomach, however, she gets off and swallows them back down. As she and Jeremy arrive at his uncle's sandwich shop, Phineas and Ferb, still inside her throat, tickle her so she will cough them back up. Jeremy goes to get her some water and they get coughed out and returned to normal size. Candace quickly calls Linda, who says she's on her way to the shop to pick up some sandwiches Lawrence ordered.
Doofenshmirtz, while wearing the belt, begins to roll himself along the wall and out through a window. Perry proceeds to do the same, chasing the doctor up building and up the giant vacuum cleaner that sits on top of it. Doofenshmirtz manages to break his belt off on the switch of the vacuum and falls down into Phineas and Ferb's submarine. Perry also breaks his belt off using the switch and turns the vacuum on. It moves forward and manages to suck up Doofenshmirtz and the submarine, just before Linda arrives.
Linda calls Candace imaginative and walks off as Isabella comes up to Phineas and tells him that Pinky threw up and she got her sash back. Jeremy then shows Candace his uncle's menu and tells her he had grilled cheese sandwiches named after her on it because he knows that's her favorite. As he hands her one, Doofenshmirtz activates the shrinking submarine and falls into it. After they go inside to eat, as Jeremy suggested, Candace is about to eat her sandwich, but Pinky jumps up and eats it instead, with Doofenshmirtz inside.
Later, while Pinky is sleeping, Doofenshmirtz wonders where he is. He learns that he's inside a dog and loudly plans to pee all over the Tri-State Area and laughs evilly, then pauses, sighs, and wonders, "Oh, what am I doing with my life?"
- Vincent Martella as Phineas
- Thomas Sangster as Ferb
- Ashley Tisdale as Candace
- Alyson Stoner as Isabella and Jenny
- Caroline Rhea as Linda Flynn
- Dee Bradley Baker as Perry and Pinky
- Dan Povenmire as Dr. Doofenshmirtz, Additional Voices
- Jeff "Swampy" Marsh as Major Monogram, Additional Voices
- Mitchel Musso as Jeremy
- Kelly Hu as Stacy
- Frank Welker as the black cat (uncredited)
- If the tacos had been in Candace's stomach since the night before, they wouldn't be recognizable as tacos anymore. In fact, they would already be digested and not in there at all. Also, they appeared to have been swallowed almost whole.
- When the submarine connects to Candace's nerves, Phineas says that they are in her stomach. However, there is no way to connect to the nerves within there.
- During the song "Hemoglobin Highway", we see a red blood cell split in half as if by mitosis. They do not reproduce in this manner. However, this may not count, as there is much biological inaccuracy in this episode.
- When the boys are building the submarine it is green, however, when it is seen inside Candace, it is blue. This could be explained as different body fluids adding a tint to it.
- In Oh, There You Are, Perry, Candace says she is allergic to dairy products, yet she is seen eating cereal with milk. She is also seen eating a grilled cheese sandwich in this episode, although it could be some sort of dairy-free cheese but this is not likely because it was made for Pinky and not her. Swampy and Dan realized this mistake in a later interview, with Dan saying simply that it was "soy cheese".
- During the song Hemoglobin Highway, we see Candace's eyeball but there is no way they can see that.
- Candace apparently does not have peristaltic action.
- Linda should've seen the submarine from a distance down the street.
- The name of this episode is derived from the novel Journey to the Center of the Earth by Jules Verne.
- The submarine that Pinky swallows is an homage to the film Fantastic Voyage.