The episode starts with Goddard making a Jack-o-Lantern and then Sheen asks what they will be for Halloween. Carl says he is going to be Llama Boy and says this year he is going to wear a cape but Jimmy and Sheen said that he was Llama Boy for the last 8 years, plus Llama Boy is not scary it's just creepy. Jimmy says Sheen is going to be Ultra Lord, again but Sheen says he wore out his costume. The three of them go into the house while Hugh is watching his favorite movie "Octopus Man". Carl asks what Jimmy will be this year. He replies that he will not trick or treat, then he makes a deal that he will make Carl and Sheen into monsters, if they give half of their candy.
They go to Jimmy's lab where Jimmy presents his Neutronic Monster Maker. Carl chooses to be a vampire, and Sheen chooses to be a werewolf. Jimmy turns them into monsters and they go to trick or treat. When Carl cuts himself in the finger with a wrapper, he drinks the blood from his finger and he says he wants more blood (as he has now turned into a real vampire). He turns into a vampire bat and flies off. Then Sheen sees the full moon come out of the clouds and tries to attack Jimmy (as he has now turned into a real werewolf). Jimmy panics and immediately flies off with Goddard in the hovercar.
Meanwhile, Hugh accidentally falls into Jimmy's lab and accidentally turns himself into Frankenstein's monster. Judy thinks he made himself a wonderful costume. He tries to grab Judy, but she goes to take her costume and gives Hugh a lighter. He quickly throws it out the door. Jimmy then tells Goddard to give him all the information about vampires (they feed on the blood of the living, can turn their victims into vampires, are repelled by garlic, and can only be destroyed by a Wooden stake driven into their heart). Ms. Fowl then appears telling him about werewolves because she was married to one. She quickly disappears into fog. Carl finds Cindy, hypnotizes her, bites her, and transforms her into a real vampire like him. While Sheen finds Libby, bites her, and transforms her into a real werewolf like him. Hugh then goes to the Candy Bar where Sam is playing his violin. Judy then comes and says she needs help at home giving out candy. Hugh does not listen, takes her in his arms, and goes off.
Jimmy then finds Cindy and Carl and finds out Cindy has become a vampire. They transform into vampire bats and chase Jimmy attempting to turn him into a vampire until they suddenly get scared and fly off. Jimmy then realizes that he is standing in front of the display window of Lucky Tony's House of Garlic. He and Goddard go into the alley where Sheen and Libby sneak up on him. When Jimmy hears growling he turns around to see Libby has become a werewolf. Libby suggests to Sheen that they should eat Jimmy, claiming that "he said they were going to get something to eat." They chase Jimmy where they also get scared because Jimmy is standing in front of the display window of the Hi Ho Silver Store. Then, Hugh, holding Judy, comes onto the sidewalk, running into Jimmy. Jimmy says that his dad probably went to his lab and turned himself into a monster. The other monsters meet up with the three on the sidewalk. Sheen and Carl attack Hugh while Cindy and Libby are fighting. Judy breaks up the fight by telling them all to work it among themselves. In addition, by work things out, the monsters conclude that they should instead scare other people. Meanwhile, Jimmy goes to his lab and turns himself into a monster, but lowering the settings so that his DNA, unlike the others', will not change.
The monsters go to the Candy Bar where Sam and Ms. Fowl call up an angry mob, armed with silver, garlic, and fire. They chase the monsters into the golf course. They all hide from the mob behind the windmill on one of the courses. They then all see Jimmy coming to them from the sky and realize that he turned himself into Octopus Man. He grabs all of them with his arms and takes them home, where he turns them back to normal. They realize that they left their candy behind, and are all very disappointed. Hugh even mentions that no candy on Halloween is scary. Jimmy says that he will fix that, and rings bells with his eight arms. Jimmy in a scary voice says, "Trick-or-Treat". A screen appears showing the words "HAPPY HALLOWEEN".
Trivia
The trick or treat baskets that Carl and Sheen have resemble things that their monster forms do.
The title is a parody of "A Nightmare On Elm Street". The title card's font is also the same as the logo of "A Nightmare On Elm Street 2".
When Carl is suggesting what costumes Jimmy should have on Halloween, he mentioned "that smart guy in the wheelchair that talks with a keyboard". He is talking about Stephen Hawking, one of the most smartest humans who ever lived.
One of the monster choices on Jimmy's wheel is Michael Jackson, who is donned in his Thriller outfit.
Miss Fowl recites lines from the 1941 movie "The Wolf Man" ("Even a man who is pure in heart and says his prayers by night, may become a wolf when the wolf-bane blooms and the autumn moon is bright."
Cindy's Halloween costume was Muffy the Vampire Annihilator, which is an obvious reference to American television show "Buffy the Vampire Slayer."
Sheen references Gene Wilder's character from Young Frankenstein (pronouncing it as "Fronkensteen").
It’s revealed Carl cuts his finger on candy wrapping.
The monsters on Jimmys’s monster maker are:
The mummy
Creature from the Black Lagoon
Octopus Man
Devil
Zombie chimpanzee
Dracula
The Phantom of the Opera
The Blob
The Hunchback of Notre Dame
Frankenstein
The werewolf
Michael Jackson
Cindy’s blood type is A+.
Goofs
The switch on Jimmy's Netronic Monster Maker switches monsters from one scene to the next despite it not being spun.
Libby is seen in her regular clothes when her and the other monsters hide behind the windmill.
Jimmy, Sheen, and Goddard waste time watching Carl drink blood, when any of them could have told him to stop drinking it, to prevent his turning into a vampire. Sheen was disturbed by it, while Jimmy just stood in shock.
It is unknown why Sam refers to Hugh as a real monster when earlier in the episode he believed Hugh had a costume on.
Ms. Fowl claims she was married to a werewolf, but in The Big Pinch, she claims to not like marriage.
When Jimmy was changing himself into Octopus Man, we see the dial on his Neutronic Monster Maker machine on the creature. However in the very next scene (where he tells Goddard to flip the switch) the dial is on Michael Jackson.
The cartridge Jimmy inserts into the Monster Maker’s control panel is already in its slot before he puts it in seconds later.
Jimmy says the Monster Maker has 102 different monsters to choose from, but the wheel only shows 12 monsters.
