Phineas and Ferb can't make up their minds whether to build a submarine or a shrinking ray. Linda says that the boys 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 Candace to spit out her cereal and milk. 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 into the backyard along with her dog Pinky to show them her sash with all her accomplishment patches. However, when she shows Phineas the sash, Pinky jumps up and eats it. Isabella gets upset because she needs the sash for the Fireside Girls ceremony that afternoon. Ferb shows Phineas their blueprints and they decide to create a shrinking submarine to retrieve Isabella's sash from Pinky's stomach.
As that is going on, Perry slips into his lair. Major Monogram tells Perry that Dr. Doofenshmirtz is hiding out in the old abandoned vacuum cleaner factory and to find out what he's up to.
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 Pinky will eat anything but dog food, though he does like grilled cheese sandwiches. Phineas says that he and Ferb will make the submarine while she makes the sandwich. When she returns with the sandwich, she finds Phineas with the completed sub. She places the sandwich on the ground, Phineas and Ferb activate the shrinking submarine and drop into the sandwich. 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 Pinky, 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 grilled cheese sandwich on the ground and decides to eat it now, 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 Candace's throat Phineas comments that Pinky's neck is really long. As they search for the sash, he notices tacos similar to the ones their mom made 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 in Candace's stomach. Ferb then states, "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 nervous system 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 Jeremy. 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. Phineas apologizes, but tells her they have to help Isabella as they tap into Candace's arms and legs. They walk her over to the monkey bars and make her hang upside down and they begin to move out of stomach through her, however she gets off the monkey bars and swallows them back down. As Candace 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 return to normal size. Candace quickly calls their mom, who says she's on her way to Jeremy's uncle's restaurant 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, as well as the tarp covering the Uncle Joe's Subs sign, just before Linda arrives. When she sees such with the boys in front, Linda calls Candace imaginative and walks off.
Isabella then comes up to Phineas and tells him Pinky threw up and she got her sash back. Jeremy then shows Candace his uncle's menu and tells her he had the grilled cheese sandwiches on the menu named after her because he knows it's her favorite. As Jeremy hands her a grilled cheese sandwich, Doofenshmirtz activates the shrinking submarine and falls into the sandwich. After they went inside to eat, as Jeremy suggested, Candace is about to eat her sandwich, but Pinky jumps up and eats it instead, with Doofenshmirtz inside.