Trick-or-treat in South Park is interrupted by an outbreak of pinkeye. While Stan, Kyle and Cartman are getting themselves ready for the school's Halloween party and costume contest, a mishap at the morgue transforms the residents of South Park into brain-eating zombies. The kids have to save the day before they can get down to the business of getting candy.
The MIR Space Station crashes on Kenny McCormick at the bus stop on October 30, killing him. He is taken to the morgue where he is drained and filled with embalming fluid, however, one of the two coroner's bottle of Worcestershire sauce falls into the embalming fluid and turns Kenny into a zombie. He breaks out of the morgue, biting the coroners on his way out.
The next morning, Kenny shows up at the bus stop as normal while the boys are waiting to go to school, oblivious that he is a zombie. Each boy is dressed up for the costume contest at school (for which the top prize is two tons of candy).
Wendy Testaburger is supposed to dress as companion Raggedy Ann, making them a shoe-in for the candy. However, she dresses as Chewbacca.
Kyle has on a store bought Chewbacca costume.
Cartman is dressed in a Hitler costume his mom made him.
Kenny is mistaken as a celebrity.
At the local doctor’s office, the morgue workers are beginning to turn into zombies, however the doctor diagnoses it as pink eye.
The kids arrive at school only to discover everyone in the class, including Mr. Hat and Wendy, have dressed as Chewbacca save Mr. Garrison who is dressed as Marilyn Monroe. Seeing Wendy as Chewbacca angers Stan, because he is now forced to go around as Raggedy Andy all day long for no reason.
Meanwhile, at the Cartman house, Eric’s mom decorates the house and yard for Halloween, completely oblivious to the zombies attacking people on the street.
Back at school Kyle, disgruntled that he has the same costume as everyone else, decides to work up a new costume for the contest while Principal Victoria and Chef are upset over Cartman’s costume. Principal Victoria makes Eric a ghost costume to amend the problem (however it ends up looking like a Ku Klux Klan robe instead). Meanwhile, Kenny attacks and bites Clyde when he walks past him.
Kyle arrives at the costume contest in the gym with a new costume, a working model of the solar system, but Wendy’s Chewbacca costumes earns her first place and the two tons of candy (which she promptly gives to starving children in Nairobi). The judge awards gives Kenny second place for his "Edward James Olmos" costume (his zombie state) and Stan "worst costume ever" for his "stupid clown thing". As Mr. Garrison says "Let's bob for stupid apples", zombie Clyde drowns Bebe who is bobbing for apples as the zombies run out of control.
Across South Park the zombies continue to attack, although everyone still believes it is simply pink eye. As night falls on South Park the boys prepare to go trick-or-treating, with Kyle now dressed as Count Dracula, just as Chef realizes that the pink eye epidemic is actually a wave of the living dead. Wendy tries to apologize for humiliating Stan, but Stan rejects her.
Chef goes to the doctor seeking help and attempts to explain what’s going on, but the doctor doesn't believe him. Suddenly, zombies break into the clinic, forcing Chef to escape out the window but the doctor gets eaten. Chef arrives for the Mayor's office to warn her and Officer Barbrady (who are up to something kinky), but they don't believe him either. As the zombies continue to ravage the town, the boys, also oblivious of the zombies, head off to go trick-or-treating. However, Kenny keeps biting everyone before they can get any treats, so the boys ditch him on the way to Chef’s house. Chef pulls the boys inside and after some discussion head to the morgue to get to the bottom of the zombie matter.
While at the morgue they discover the bottle of Worcestershire sauce as well as an issue of "Crack Whore Monthly" magazine with a picture of Cartman's mom on the cover. The zombies (led by a zombie version of Pip) eventually find a way into the morgue and turn Chef into a zombie (who then goes into Michael Jackson's Thriller mode). The zombies then pursue the boys, who are forced to fight them off their chainsaws (except Chef, who is busy reading "Crack Whore Monthly"). Through the commotion, Kyle calls the hotline number on the bottle of Worcestershire sauce for assistance. He is connected to an operator who tells Kyle "not to kill zombies willy nilly", but to kill the original zombie (Kenny) in order to turn all the other zombies back into humans. Wendy arrives as a zombie and lurches towards Stan he can't bring himself to kill her. She is returned to normal when Kyle quickly cuts Kenny in half with a chainsaw, killing him. All around them the dead limbs of the townspeople begin to turn back to normal flesh ones.
Cartman cries at Kenny's grave while with Stan and Kyle. However, he then stops leaves to count his candy. Stan and Kyle follow him, saying they're going to his house to find more naughty pictures of his mom. Cartman defends her by saying his mom was young and needed the money, even though the pictures came out a month ago. After they leave, Kenny comes out of his grave all stitched up but a statue from another tombstone crushes him, then a jet smashes into it.