Timmy and his friends' Halloween costumes are very terrible, so he wishes that everyone's costume was real. However, this wish causes the popular kids to become evil and morph into the evil Jack-O-Bots who begin a countdown to destroy Earth! Meanwhile, the Yugopotamians see the humans celebrating Halloween, and fearing an attack they launch an invasion of Earth.
Timmy is watching the "Crash Nebula Halloween Special" on television. In the show, Crash Nebula narrowly escapes an evil pumpkin shaped robot called the Pumpkinator, as it destroys the pumpkin-shaped planet they were fighting on. Although Crash Nebula escapes with his life, he screams in frustration as the Jack-O-Bots, four robots that come together to form the Pumpkinator, escape and fly off through space. Timmy points to the television as it shows the Jack-o-bots, telling his godparents that is what he wants to be for Halloween. Cosmo takes this literally and turns Timmy into his television that he was pointing at. Meanwhile, Mr. and Mrs. Turner have decided to go as each other for Halloween, so Mr. Turner has his wife's voice and vice versa. Wanda explains to Timmy that only four Jack-o-bot costumes were made, and they were all won in a contest. As Timmy asks who, the doorbell rings and the first trick-or-treaters of the night appear at Timmy's doorstep.
The visitors are Tad, Chad, Trixie Tang, and Veronica, the popular kids, who were dressed in the Jack-O-Bot costumes Timmy wanted, having won them unfairly by bribing the judges. Chad presses a button on his suit, which causes an egg and toilet paper launcher to appear from his Jack-o-bot suit. Not wanting to be tricked, Timmy's parents hand the popular kids all of Timmy's stuff, his video games, his CD's, and his birthday present money. As the popular kids walk off, Timmy asks his parents why they gave all his stuff away, and they answer that it was because the costumes were so good. Chester and A.J. show up, but their costumes are not as good, so they just get papers with useless advice written on them instead. Chester and A.J. complain to Timmy about the "treats", who responds that they are not going to get any treats in their lame costumes. Offended, they ask Timmy what he is going as, and he jokes "the scariest monster of them all... Vicky!" He laughs, unaware that Vicky is standing right there. She has been hired to watch Timmy and his friends while they go around the neighborhood, trick or treating. Vicky grabs a roll of toilet paper and wraps Timmy in it, giving him a really makeshift mummy costume. Chester and A.J. briefly laugh at Timmy's lamer costume, until Vicky explains that she is getting a 70/30 split of all the candy, and tells them to get moving. Chester and A.J. dash off, dragging Timmy behind them.
While the group walks along the sidewalk, Timmy notices Cosmo and Wanda following him in fairy form. They are disguised as a "floating human janitor" and "floating human nurse" respectively. Timmy asks them fearfully what they are doing, because if anyone sees them they will go away forever, but Wanda assures him that because it is Halloween, no one will notice the difference. Vicky comes over and dumps two empty treat bags on the newcomers, telling them to start getting her some candy. Meanwhile, someone is watching them from above in a spaceship. It's King Gripullon and Queen Jipjorrulac, rulers of Yugopotamia. They see all the costumed people walking around and think that Earth is hosting an intergalactic convention, without inviting the Yugopotamians. Fearing a conspiracy, the King decides on a royal response of the highest order... pouting.
Timmy and the others arrive at Principal Waxelplax's house. They watch the popular kids walk by, holding bags stuffed with candy, and remarking that the Jack-o-bot suits actually give them enhanced strength to carry the heavy load of candy they are receiving at everyone else's expense. Principal Waxelplax, who is dressed like a mirror for Halloween so all the students can see how "wonderful" they look, is giving out safety items such as reflective tape for Halloween, no candy. Chester pulls out a roll of toilet paper and raises his eyebrows to A.J., but Principal Waxelplax warns that if her house gets toilet papered, they'll all be held back a year. Chester says he is going to be held back a year anyways, and decides to go wild. At the next house, a scary looking Victorian-style house, live the Benders. Dr. Bender is dressed normally and Wendell is dressed as a tooth fairy, when Timmy remarks "nice costumes" they both say "What costumes?", Dr. Bender prepares to give them sugary candy, then pulls out his dentistry tools and warns them he'll be "scraping them off your gums in no time!", Timmy and his friends run off in horror, while Wendell tells his father he is why he has no friends.
Back in space, King Gripullon notices Vicky eating a piece of candy, and realizes that the Queen was correct and that the Earthlings are conspiring against them. The two aliens believe that Earth is arming itself for war by stocking up on candy, a substance very harmful to Yugopotamians. The King prepares a response of an even higher order... to write an angry letter. At the Buxaplenty Mansion, Timmy watches Mr. Buxaplenty give the popular kids outrageous treats, such as twelve thousand dollars in cash, or a wheel-barrel full of gold coins. When Timmy and his friends ring Mr. Buxaplenty doorbell, he chooses between what type of dog to unleash on the kids. He selects "pitbull" and a pair of brown pitbulls chase Timmy and his friends off the property. Timmy's toilet paper costume gets caught on a branch and he falls, he yells out to the others "Go on without me!" and they happily oblige, leaving Timmy to be mauled by dogs. As the other kids give their candy to Vicky, Timmy realizes that the popular kids are getting candy because their costumes are real, and Vicky is getting everyone else's candy without her own costume because she is scary.
Timmy wishes for everyone's costume to be real and scary, which is present in the form of a song. Timmy becomes a real mummy, a walking rotting corpse. He arms frequently snap off whenever he applies pressure to them (such as hugging Cosmo and Wanda). Meanwhile, Tad and Chad come across Elmer who is dressed like Crash Nebula. They joke that Elmer has "intergal-acne" before the wish effects them and the three boys become their costumes. Crash Nebula/Elmer tries to face off against the two Jack-o-bots Tad and Chad morph into, but his "pulse pounding pulsing face pustules" are no match for the two Jack-o-Bots when they merge together, and Elmer is frozen by their ice gun. The Jack-o-Bots declare that they will form together and make it the worst Halloween ever, by counting down and destroying Earth.
Vicky demands that Timmy find her more candy because she is low on sugar. He notices that Vicky is unaffected by his wish because she always was a monster anyways. Wanda says that all the other kids are enjoying their costumes at least, which surprises Timmy because he did not expect his wish to affect everyone. In outer space, Mark Chang and his friends, Jeff and Erik, ask the King and Queen what they are watching. The King and Queen say that the humans have triggered a doomsday device which will destroy their entire planet (the Jack-o-Bots), which Mark thinks is awesome, until he realizes that Vicky is down there as well. Mark asks his father for permission to go to Earth, which the King agrees because they may want to preserve a single specimen of the human race once the others are wiped out. Mark and his friends exclaim "ROAD TRIP!" and fly the space ship to Earth, they disembark from the spaceship in special suits designed to protect them against candy. Mark tests it out by grabbing a candy with a metal claw and handing it to Jeff, the candy gumdrop burns right through his suit and tentacle. Mark pulls out a Vicky detector and tracks her down, although she was only a few steps away from him anyways.
Noticing Mark and his friends, Vicky asks "What do you geeks want?", still unaware that Mark is really a space alien. Erik sees Vicky holding a bag of candy (with Timmy's severed arm attached), yells "She's armed!" and knocks the bag of candy away, raining down more painful burning pieces of candy onto Jeff. The bag lands near Timmy, and Wanda tapes his arm back on. Timmy notices that even "those kids" have better costumes than him, authentic Yugopotamian ones, until Timmy realizes that only him and his fairies know about the Yugopotamians. When Timmy and his fairies hear Mark exclaim "I LOVE YOU!" to Vicky, they confirm that it is really them. The overhear Mark ask Vicky to join in escaping this "doomed planet". Timmy looks up and realizes that the Jack-o-bots are real now and forming together. Timmy tries to wish everything back to normal, but Cosmo and Wanda have become real and scary janitor and nurse monsters... with no magical powers. Timmy tells Cosmo to keep Mark distracted and on Earth, while Wanda to roll his head to the nearest costume shop (Timmy's mummified body has completely fallen apart).
Mark presses a button on a pair of keys he was carrying, unlocking his spaceship with an alarm chirp. Before he, Vicky, and his friends could board, Cosmo jumps on the ramp in front of them and tells them he is there to inspect their vehicle... after his break. Cosmo pulls out his pillow and starts napping on the spaceship's entry ramp, blocking access. At the costume emporium, Wanda rolls Timmy's head to the entrance, but it is locked. They noticed the two of the Jack-o-bots flying above them. Trixie shoots lasers at Timmy and Wanda, who narrowly avoid them, and almost kills them, and now an entrance has been blown in the wall to the costume store. Timmy enters the costume store while all four of the Jack-o-bots form together, forming the Pumpkinator, who begins a countdown of five minutes until he blows up the Earth. In the costume store, Timmy tells Wanda to throw him into the bargain bin, as one costume will not be enough to defeat a thirty foot robot, but a combination of several might do the job. Timmy emerges as a Crimson Chin/Crash Nebula/Vampire/Lagoon Monster/Ballerina hybrid and squares off against the Pumpkinator. During the fight, Mark Chang's spaceship blows up as the King and Queen watch from space. They believe Mark has been killed, so they decide to mourn him for "9000 Yugopotamian days", about a second later the mourning period is over and the King tells his guard to unfreeze one of Mark's clones. On Earth, the battle continues...
Timmy and the Pumpkinator continue to fight, there is only about a minute left. Timmy tries to use his vampire power to transform into a bat and bite the Pumpkinator, turning it into one of his undead slaves, but it does not work and Timmy loses a sharp vampire tooth which falls to the ground next to Cosmo and Wanda. Wanda grabs Cosmo's pillow and puts it ontop of the giant tooth, causing a tooth fairy (Wendell) to appear. Wanda pushes him aside and snatches his wand and crown, using them to transform back into a fairy. Timmy wishes that everything was "fake and safe" and the wish is granted. Everyone is back to normal. The Pumpkinator's countdown stops at one second before the wish turns the popular kids back to normal, and they land in a pile on top of each other, and drop all of Timmy's stuff that had been given to them earlier. Mark notices that Timmy is not a real mummy, but that "stupid Earth kid he runs into every time he comes here". Timmy realizes that Mark and his people do not know what Halloween is, and tricks them into thinking Earth has declared war on Yugopotamia. The King and Queen, relieved to see their son is alive when he transmits a distress message, teleport him off the surface of the planet. Mark is momentarily saddened to leave without Vicky, but quickly cheers up when he learns a Vicky clone has been created for him in the mean time.
The Yugopotamians launch a "P-Bomb" at Earth and then fly off back toward their planet. Wanda asks Timmy why he tricked the Yugopotamians into bombing Earth, and he replied because they were idiots who think that sweet stuff is bad, beautiful is ugly, and their ultimate weapon of destruction, the P-Bomb, is a giant piñata. The bomb explodes in the atmosphere, causing candy to rain down on the cheering children below. Now every kid has candy, regardless of whether Vicky stole theirs or if they had a terrible costume. Vicky is sick from all the candy she ate, and never wants to see a piece of candy again. A huge pile lands on Vicky and she screams "It burns!".
A few weeks later, a huge line of kids is forming at Dr. Bender's Dentistry Clinic. All the kids have cavities, but are otherwise happy that it rained candy and declare it the best Halloween ever. Dr. Bender gives the children more candy, hoping to continue to profit from their cavity filled teeth. Cosmo after reading a magazine says to the audience you can't spell Halloween without ow in the middle. He eats a candy cane but it breaks his teeth and he responds with an "ow". The episode then irises in on him.
The Yugopotamian P-Bomb rains candy on Timmy and his friends.