In order to study about the human body, Cosmo and Wanda shrink Timmy so that he can explore Vicky's human body, but chaos soon ensures when Vicky's body attacks him and becomes separated from his godparents, while Vicky throws a party to trash the house!
Timmy has a project due for school on the human body, but he finds his interest in studying cells to be "microscopic". Downstairs, Mr. and Mrs. Turner show an expensive vase to Vicky, and warn her not to break it, as the 50,000 dollar vase is worth more than she'd make in her entire life. This angers Vicky, who glares at Timmy's parents while they leave in laughter. Timmy is looking outside at the pouring rain, muttering in boredom about his school project. Cosmo and Wanda wish him up a shrinky suit to spark his interest. Timmy eagerly tries out the suit, and it shrinks him down to tiny size, although he quickly becomes bored again. They use the suit to explore around the house a bit, until they discover a table full of junk food. Timmy discovers a protozoa, and, thinking he has things under control, gives Cosmo and Wanda the afternoon off after remember what destruction they caused to his house when helping him with his last science project.
Timmy shrinks down to microscopic size and "interviews" Princess Protozoa, a snobby microscopic creature that likes Timmy because she finds multicellular organisms complex. Meanwhile, Cosmo and Wanda, who happen to be snacking on some "Cheeze pantz", accidentally get eaten by Vicky then she lets out a big burp, and go on a tour of her digestive track, along with three germs, who claim to be "The Stomach Flu". The flu germs cause Vicky to suddenly need to puke (by touching some villi with the tour guide saying they have to keep their hands in the boat at all times), and while running towards a bathroom, she knocks over the Turner's prized vase. She decides to blame this on Timmy, and calls some teenagers to come to Timmy's house and break things, claiming it was Timmy's party. Meanwhile, after abruptly finishing his "Interview" with Princess Protozoa in part thanks to his short attention span, Timmy finds out the house is being demolished, and tries to return to normal size. However, Cosmo and Wanda forgot to add a "Grow" option to the Shrink suit, so Timmy shinks a bit smaller. He then finds the Cheeze Pantz bag in the garbage, and discovers that Cosmo and Wanda were eaten by Vicky. Timmy then proceeds to enter Vicky's body and find his fairies.
Cosmo and Wanda are finishing up their tour and come across "Kidney Land" and Walt Kidney. Meanwhile, Timmy is searching for Cosmo and Wanda, only to find a Black hole in Vicky's body, located where her heart is supposed to be. Timmy realizes her heart IS a Black hole, and gets sucked inside. The Black hole takes Timmy to Vicky's brain, and he starts snooping around, hoping to find Cosmo and Wanda. Timmy finds Vicky's brain to be "very vacant", and enters a room with many small men on computer monitors that represent parts of Vicky's subconscious. Timmy encounters Vicky's anger, which is the form of a cute innocent looking girl who asks Timmy if he would like a cookie, and when he replies yes, she screams "SO WOULD I!" in his face like Vicky would.
Meanwhile, Cosmo and Wanda have finished their tour at Vicky's nose, which is also the exit for the tour. Cosmo, Wanda, along with the stomach flu germs, all get sneezed out of Vicky. One of the men in Vicky's subconscious notices a girl getting more attention than Vicky, so he makes her spill something on the girl's dress. Timmy turns up Vicky's kindness, causing her to give most of her money to the people breaking down the house, and dress similar to Disney's Snow White while she calls in her "Animal Friends" to clean and fix the house. While Timmy watches her do this through her eyes, he notices Cosmo and Wanda as she cleans up her snot from a picture.
Realizing he can leave now, Timmy begins to make his way out of Vicky's brain, when the brain cells figure out what Timmy has done and stop him. They chase him out of the brain and send anti bodies after him after he escapes in his Shrinky Suit. Timmy makes his way to Vicky's vocal cords, and finds the voice box, which looks like a drive through speaker. He speaks "I WISH I WAS OUT OF HERE!" through the voice box, making Vicky say those exact words, with Timmy's voice. Cosmo and Wanda recognize this voice as Timmy's, and poof him out of Vicky just before he is destroyed by the Anti Bodies. The head brain cell switches Vicky's kindness back from full power to off, causing Vicky to force the animals out of the house. However, everything except for the vase is repaired, which Timmy fixes with a simple wish. Timmy's parents return home shortly afterward to find the place spotless. Timmy, due to his short attention span, accidentally knocks over the vase, causing it to break. Vicky gloats that Timmy is about to get in trouble, but Timmy's parents exclaim that they are now rich(er). With both kids confused by all this, Mom and Dad explain to Timmy and Vicky that the vase was insured for $50,000, but they only bought it on the Internet for one dollar.
The next day, Timmy is presenting the inside of the human body, using what he discovered from within Vicky in his report. Mr. Crocker fails him for making this all up, knowing full well that is not what is in the human body. The episode ends with Timmy having to study the 'Big Universe', to which Cosmo and Wanda make Timmy SO BIG he can sit on the Earth. He tells his godparents that they take everything too literally as the episode ends there.