Later that morning, Zoe is excited about her birthday party that day, especially because a boy named Steve, whom she has a crush on, is coming. But suddenly she starts feeling weird and spots appear on her face. Her mother instantly tells her that her party will have to be canceled – if she has a fever. She does end up with a fever.
Max had peeked in on the situation, and he comes to the conclusion that Nasal is inside Zoe. He suggests to A.B. and FX that they go in after him, so FX turns them small. Meanwhile, Zoe's fever seems to have gone away, so her mother lets her have her party after all. She does, however, instruct Zoe to take a cold capsule, which Max and company hitch a ride on to get inside Zoe. After they get inside, FX turns the capsule into a submarine of sorts.
Zoe greets her party guests while Max and friends go to her stomach to ask people if they have seen Nasal. The "supervisor" there tells them that Nasal had been there trying to start trouble. He doesn't know where Nasal ran off to, but he suggests they look in the Heart because the leader up there knows a lot. But the supervisor spends too much time talking to Max and not enough time concentrating on his work, causing pressure to build up in the stomach and eventually making Zoe burp in Steve's face.
Max and the gang leave the stomach and travel to the heart via Zoe's throat, but while in the throat pressure builds up again – at the same time Steve offers her some of his mother's homemade cookies. She tries a cookie, but because of the pressure, she keeps coughing. The gang gets out of the throat and makes it to the "Heart Club", where they learn that Nasal had run off to the lungs.
They get to the lungs where they find Nasal, who is hoarding a dozen(?) of special eggs. He explains that all Zoe has to do is sneeze a single time and the eggs will hatch, spreading viruses all over her body. There is also a mountain-climber-type virus climbing her nose, trying to get her to sneeze. Max goes after him while A.B. and FX head off for Zoe's brain to suppress the sneeze reflex.
The mountain climber starts to tickle Zoe's nose with a feather, but Max gets rid of him before he can do any real damage. He then joins A.B. and FX at the brain, only to learn that they haven't found the sneeze reflex yet because there are too many buttons and levers there to figure out what does what. Zoe is about to sneeze in Steve's face, so the gang frantically presses several buttons, which in turn causes Zoe to stick her face in the punch bowl for, as Steve says, bobbing for punch. The other party guests follow suit.
Zoe and her friends start dancing, while Max decides that the next plan of action is to go after Nasal again. But Nasal arrives first with a horde of little germs, and FX makes a golf club (and eventually a cue stick) for Max to knock them out with. Because of all this commotion, Zoe starts dancing uncontrollably. After his little germs are defeated, Nasal runs out.
The gang finds Nasal back in the lungs with his eggs, and, thanks to FX, they wind up in an airplane, where they dump chicken soup on the egg viruses, making them disappear. Max then jumps out of the plane and ties up Nasal. Before completely exiting Zoe's body, they make a quick stop to her eyes just as her party guests are leaving. Steve tells Zoe that he had a really nice time, and then he kisses her on the cheek and leaves. Tears well up in Zoe's eyes and they eventually run out, bringing the gang and Nasal with them.
When they get back to Max's room, FX makes everyone (except Nasal) big again, and then Max calls Wally. When he arrives, Max gives him a baby bottle that has an enraged Nasal in it. Wally is grateful and tells them that they can go into space anytime. After he leaves, Zoe runs in cheering about her party and how Max didn't ruin it for her. Max winks at the screen.