This episode starts out with a little girl telling the class why she's a fairy princess for halloween. Ms. Bitters said that she was a fairy princess for halloween once too, until she flew into a bug zapper. She said that halloween is shamlessly commercial ploit to turn children into candy starved zombies. Zim gets scared at this and looks at some kids in their costumes giving him a dirty look, and he gets scared when Dib isn't there. Ms. Bitters asks what was wrong, and he says they've taken Dib and drained him of his sweet blood candies, and she asked who has taken him, and he says the candy zombies. Dib is seen running through the halls, screaming, but the halls look a little weird. Zim told her it's like she said, the children try to end the hunger for sweetness in their rambling, undead bellies, and Ms. Bitters told him that's not what she said. Dib runs in panting, and Zim seeing this, screams and hides under his desk. He tells Ms. Bitters he's having horrible nightmare visions, and she told him it's called life. The girl sitting behind Zim said that Dib is acting more crazy today, and Zim told her that it was starting, hes one of the zombies now, and the girl beside Zim told her that her that she thinks Zim's being kind of wack, and Zim started hissing at her. The girl behind him says that Dib does have a long history of crazy, and for all she knows, Zim's just weird, and a boy says Dib's screaming like a howler monkey, and that seems more weird than crazy to him, and she said that he's always screaming like a howler monkey. Dib looks at his arms, and they start to disappear. The whole class gets to rambling, and Zim looks at Dib and he starts to disappear. His whole body disappears, and no one notices but Zim, who is going crazy, and he suddenly reappears. Dib starts screaming about hideous monsters, and one of the girls asks Ms. Bitters if they could use a crazy cards to send him to the crazy house for boys, and she tells her that each class only gets three crazy cards a month, and asks if she's sure she wants to use one, and she said yeah. She put a crazy collar on him, and told him the white coats would be there any moment, and they knock a hole in the wall where the door was, and while they got him, Zim slipped out, and then they made another hole in the wall going the opposite direction. Zim said that they'll soon be after him and his delious candy guts, and he must prepare his defenses for halloween madness.
The white coats out Dib in a padded truck with a cage that sees to the cabin. Dib tells them that they don't understand, and the woman said that if there's on thing they understand, it's insanity. Dib disappears, and shows him a parallel dimension, which is a destroyed version of what he's living in now. The two new people driving the truck are saying that he's the one, the one with the flashing collar, and they said she said that he'd come, and he has. Dib asked what was going on, but the parallel white coat, now a monster, said for him to shh! because he's talking all spooky. Dib disappears back into normal earth, and asks who's been waiting for him, and the driver asked what he was talking about. He was about to tell them what the monster said, and then said nevermind, and said that he was sane.
At his base, Zim tells GIR that they have to prepare the base for the horrible Earth holiday, but GIR is playing with an octopus, and stops when he says his toquitos were burning, and runs into the kitchen.
At the crazy house for boys, a docter is interrigating him about his reality jumps that he's been having. He tells him that last night he was in his dad's lab making some modifications to his dad's dimensional scope, wanting to peek into spooky alternate realitys, and also wanting cool costume ideas for halloween. He had theory that some paranormal phenomenons might be a result in collisions between our world and other dimensions. He tripled the output, and he couldn't really see much, just a brief flash of something, like some sort of malfunction, but since then the flashes keep coming, even without the machine, getting longer each time. Professor Membrane says he knew it would be a matter of time when he got there, and told him that he knew not to use his tools for parascience, and everyone starts muttering to each other. Dib tells them that if the jumps keep getting progressivly longer, it will only be a matter of time before he gets stuck. The doctor tells him that he's got some bad news, and says that they've decided he was insane, and he'll be confined there for a period no shorter than the rest of his life. The white coats take him to floor thirteen and throw him in a padded cell. He suddenly has another flash. He slides down something, and almost gets eaten by a monster. One monster tells him that he's the one, and he tells him that he's wrong, but the monster recites the prophecy, "The boy with the flashing neck will come, he will be the key to freedom... and stuff," and Dib asked it that was Nostradomus, and the monster told him that he read it off of a bathroom wall. Dib says that he's got to get out, and the monster tells him that he has to escape his own head, because they were in his head. Dib told him that he's insane, and the monster tells him he his, but it's true, the whole dimension exsists in his mind, and maybe that's why his head is so big. Dib thought that he might be crazy if that world was a result of his imagination. A parallel white coat tells Dib that it's time to go, and a straight jacket comes and puts itself on Dib. Dib has another jump right before they put him in the truck.
In the other dimension, Dib is running down the street, seeing Zim's base, and says that it's the only chance.
Zim is boarding up his windows and doors, while people start to run down the street to people's houses. Zim tells GIR that they've mutated, when a hole bunch of kids come to Zim's house, breaking through the boarded up windows, and Zim tells GIR to activate the defenses. A whole bunch of machine guns pop out and start firing on the kids. Suddenly, Dib burst throught the window that Zim was near, and Zim yells for him to get away from his blood. Dib is trying to says something, but he tells GIR to get into defensive mode, and he jumps out of the window. Dib tells him that the next jump might be the last, and he says that they're going to do something evil in his head, when someone yelled trick or treat, and Dib yelled that his head wasn't big to Zim. Zim told him he didn't say that, and Dib told him that he's the only on that can help, because his dad would let him use his equipment, and he was the only person with the technology, and Zim told him to go, that he's had enough of his nonsense, from his smelly mouth filled with... corn. Dib tells him that he hasn't been eating corn, and Zim calls him a liar, and tries to push him out of the window, but Dib has another jump, bringing Zim along with him. In the other dimension, a monster said they've been expecting him, and Zim yells that they're the halloweenys, and runs out of the house they were in.
Zim and Dib are running down an allyway, and Zim asked where they were. Dib told them they were in some kind of alternate universe, based on his imagination somehow, and Zim got mad because he brought him there, and put on his spider legs. Dib told him the only way back home is through his head, and if anything happens to him, he's stuck there forever. Zim told him that it was his imagination, and to find a way out of it somehow, and Dib told him they couldn't go back Zim's, so they were going to his house because there might be a version of the dimensional scope there.
Zim and Dib start running to where Dib's house should've been, when they stop and see monsters walking down the street. They hide behind a tree, but one of the monsters see's dib flashing collar, and they start running towards him. Dib rips his collar off and puts it on Zim, and drags him out to where the monsters where and told told them that he was the one in monster voice. One of the monsters is a cage monster, but before Zim could run, a tube came from him and sucked him into the cage. Zim yelled that he would get him.
The monsters take Zim to where Skool would be, and Zim was tied up to a chair, with something pointing at him. Zim told them to let go of him before something horrible happens to him, and one of the monsters walked up somewhere and told someone that they brought him, his neck flashes and everything. A parallel Ms. Bitters looks at Zim and tells the monsters that he was the wrong one, and she condemned the head monster to the Real of Eternal Screaming and Restlessness, and opened a door, where fire came spilling out. The monster said he really didn't wanna go there, and she told him that he would have to accept it, and gave him a ticket and a suitcase. Before he walked in he told them that he'd see them around, put on a hat, and walked in. Parallel Ms. Bitters said that Zim would prove useful.
Dib was walking down the street when a monster came down the street, saying that they have his friend and they will destroy him if he doesn't surrender his head to them, and Dib yelled out that Zim wasn't his friend and they could keep him.
At Skool, the monster that said that came and told Parallel Ms. Bitters what he said and she sent him to the same place that she sent the other monster. While everyone was seeing him off, Zim escaped. He was talking to himself about Dib, and he said that he hoped poor GIR was alright against the zombies, and it showed GIR going around stealing peoples candy.
Dib walked into his parallel house, and sees his parallel dad. He tries to run, but he extends his arms and grabs him by legs, making him trip. A Parallel Gaz tells him that they were going to open his head, and Parallel Professor grabbed Dib by the head. Zim makes it outside of Dib's house and hears Dib screaming, and hides when Parallel Gaz and Professor Membrane comes out of the house. Zim gets mad because he just escaped and has to go back, but he has a plan first. He goes into Dib's house and looks for the scope, finally finding it.
At Skool, Dib is in the same place that Zim was, and the laser thingy pointed at Dib. Parallel Bitters is saying that they have another world to ravage, and once they open the portal in Dib's head, the fun will begin again, and the monsters started cheering. Parallel Bitters said, "Let the Head... Thingy... Start!" and the laser started to go against Dib's head. Parallel Bitters aid that her army will soon march through his disturbingly large head, and move on to victory. Dib started thinking that his head might be big, when Zim crashed through in some robot thing, and he picked Dib up. Dib asked if he was still mad about the leaving him to rot thing, and Zim squeezed him, implying that he is. Zim told him that he wasn't there because he liked him, he was just there because of his fiilth gargantuan head, and Zim started spraying all the monsters with slime stuff. Parallel Bitters got angry and mutated, getting larger. Zim took Dib and ran, and Bitters told her army to find them, but they all just sat there.
Zim put Dib down, and told him that he set the robot thing for auto defense, and he was going to use the scope to widen the portal in his head. Dib told him that he didn't know how to use that, and Zim told him that he's used more complicated devices as play things, and he'll figure it out. It shows Zim shoving the scope in Dib's head. The robot exploded, and the shadow of parallel Bitters was walking through the smoke. Zim said that should be wide enough, and Zim said that was a good question, but he didn't care, and he jumped through the hole in Dib's head.
In regular world, kids are shown laying down, and GIR, really, really fat, laying in a huge pile of candy, when a hole in the sky opened and Zim came shooting out, landing near his house. In Dib's world, parallel Bitters was about to jump through the hole, when Dib put his legs in the hole, coming out of the portal inside out, but getting back regularly when he landed. There was still a hole in his head, when parallel Bitters starts coming through the hole. She gets her head out, and sees all the kids lying down in their costumes, and gets freaked out, and finally sees the fat GIR, and decides to go back into her world, and the portal closes.