Satan is throwing the biggest Halloween costume party ever and no one will be admitted without a wristband. Satan is busy checking the R.S.V.P. list and deciding what costume to wear to the big event. Every detail must be perfect for the prince of darkness. But even Satan can't foresee everything. Meanwhile, Butters summons Biggie Smalls.
Satan decides that since Halloween is essentially his holiday, he is going to have a huge party in Los Angeles— as big as a sweet 16 party. He proclaims to the spirits of the damned that anyone who wants to get in must get a blue wristband, wear a costume, and that not appearing will make you a loser. He also states that no one should come dressed as The Crow as that's his costume but later he makes his minion Zazul swap his schoolgirl costume with him, yelling "No one can look hotter than me!" He also invites many celebrities, and plots to have it become the greatest party ever by arranging a really huge surprise for dessert. At first, he decides a giant chocolate fondue fountain would be a good surprise, but changes his mind when he learns that Diddy already had one when he had his birthday at the hotel. His other ideas are a doughnut machine and a full ice cream bar. When he finds Diddy had them all, he decides upon a Ferrari Cake (a cake the size and shape of a Ferrari Enzo), which three notorious serial killers — Ted Bundy, Jeffrey Dahmer, and John Wayne Gacy — are ordered to bring. Meanwhile, the Roman Catholic Church, angered that Satan doesn't invite them to the party, plans to call the fire marshal the night of the party to complain, thus ruining it.
The South Park boys experiment with a Bloody Mary-type ritual to summon Biggie Smalls and Butters succeeds in summoning him. However, Biggie already has a blue wristband and, since he is on Earth and cannot use the inter-dimensional vortex opened from Hell to get to the party, demands that Butters help him get there.
Finding out that Butters disappeared, the kids try to see if the Biggie Smalls trick works and do the mirror summon; Smalls, who was sitting next to Butters on a plane bound for Los Angeles, teleports against his will back to South Park. In a rage, he begins shooting the boys. He is later summoned against his will, once again, by Randy, Stan's dad. The Catholic Church tries to call the fire marshal, but to their shock, they find that Satan, realizing someone bitter about not being invited would do exactly what they're doing, has invited him — so they decide to crash the party. However, Satan's Security makes sure they don't get in at all, as they're wearing wristbands from Knott's Berry Farm.
The three serial killers run into trouble trying to pick up the cake: having killed the bakers. This is compounded as every time they enlist someone's help, Gacy (and once Dahmer) kills them and Dahmer performs necrophilia and cannibalism on the body. Ted Bundy's attempts to discipline them only slow progress down further. When an attempt to lift the cake fails, resulting in its destruction, the killers know they're in trouble. When trying to bake a new one, the three mutilate each other Gacy poked Bundy in the left eye, Bundy sticks his first finger and pinky finger into Gacy's eye rises and forces his eyes right out, leaving him with bleeding sockets, Dahmer stabs Bundy right through the ribs Dahmer sticks his tongue out at Bundy twice, Bundy grabs Dahmer's tongue and pulls it right out of his throat, Dahmer pukes out blood then falls down Gacy looks to his right and left wondering why he see's darkness Bundy wakes Gacy with Dahmer's tongue Gacy trips then he goes down Bundy goes down as well after realizing he still had the knife stuck in his ribs and his blood loss. So Satan's assistant Demonius, who was in charge of planning the party decides to search for a new one on his BlackBerry and finds a cake the exact size and shape of an Acura TSX, which he decides is an adequate last minute substitute.
Unfortunately, it isn't good enough for Satan (though it is for his guests). Satan flies into a rage over the Acura cake, screaming that the party is "Not about them, It's all about MEEEEEEE!!!!!" despite everyone having fun. The party goers get upset, and Satan realizes that, in trying to have a party LIKE the girls on My Super Sweet 16, he has BECOME like one of them, and even when his minions insist he is not nearly as bad as they are, he insists he is. He tells the crowd that he is sorry and that his party is for all of the people who make his life so special ("And if I don't realize that, then I'm no better than a rich American teenage girl"). Now he wants everyone to enjoy his party so he invites everyone in, including the Catholic priests. Butters makes it to the party and uses a makeup mirror to summon Biggie Smalls straight to the party, who asks Butters to come into the party with him as thanks. Butters quickly agrees, seeing as "I'm gonna get grounded anyway, I might as well." The episode ends with everyone dancing, including the Catholic priests with naked altar boys on leashes who are also trying to dance.