The episode begins at nighttime, with Slimer convincing a reluctant Janine, who is working late, to make a paper plane. When it crashes, he makes one of his own, and despite looking shoddy, it flies. He answers a knock at the door, only to slam the door and fly back in fear when it apparently turns out to be a demon. Janine puts a proton pack on and prepares to shoot him, but it turns out to actually be a man in a costume. He's a singing telegram, who raps an ad for a spooky theme park called 'Ghostworld', inviting Janine, Slimer, and the Ghostbusters to a private tour the next day.
The Ghostbusters arrive home and Slimer shows them the invite. Ray accepts, but Peter and Winston aren't interested, and Egon declines because he appears to be coming down with something.
The next morning, Ray, Janine, and Slimer go to Ghostworld and meet the owner, Carl Sands, who knows their names. He gives them a ride around the park, where you shoot at what appear to be flying ghosts and demons with a type of weapon called a "ghost bopper". Eventually, the ghosts steal the "boppers", however, and Sands morphs into a demon.
Ray, Janine, and Slimer arrive home and insist that Winston and Peter go to Ghostworld with them, even though it's nighttime and they're busy with chores. They get a call, so Winston and Peter leave (while Egon is still in bed because he's sick with a cold) and then Ray, Janine's, and Slimer's eyes change colour and they (except Slimer) grow fangs, revealing them to be possessed.
While Peter and Winston are attacking a fish ghost, the possessed Ray and Slimer show up and pretend to try to shoot, while actually letting the fish ghost escape. They claim Ray's proton pack isn't working, then everyone gets into the car and drives off. In the car, they convince Peter and Winston to drive them to Ghostworld by pretending they're making a TV special.
They arrive there and are greeted by the still-possessed Janine, then they go down a ride and meet Sands in demon form, who wants Egon to show up since he wanted all four of them. When Janine tells him that Egon will arrive for the others, the demon explains that the ghosts at the park are real and are looking for humans to possess. Peter and Winston try to escape, only for the demon to catch them with some kind of force field from his finger.
At the firehouse, Egon's mother has come over to nurse him back to health against his wishes. He sees the TV special, where Peter and Ray are now possessed too, and realises something is up. He drives off to save them, with Mrs. Spengler tagging along against his wishes.
The Spenglers find Janine, who asks them to follow her, and Egon determines she's possessed when she believes his lie that Mrs. Spengler is his aunt. Janine turns into a monster and the Spenglers use a ghost trap to exorcise her. Janine explains what's going on and Egon reveals that the demon's real name is Karro Zans. Zans shows up and he and Egon insult one another. Egon goes all around the park and Zans searches for him, while Mrs. Spengler and Janine exorcise Ray, Winston, Peter, and Slimer. Eventually, everyone works together to trap Zans.
Back at the firehouse, Egon is better, but Ray, Winston, Slimer, and Peter have caught his cold.
This episode provides examples of
- Accessory-Wearing Cartoon Animal: The fish ghost wears a pirate's hat.
- Amusement Park of Doom: The amusement park houses evil entities that end up possessing several characters.
- Aside Glance: The episode ends with Egon looking at the viewer.
- Beneath the Mask: In his human guise, Karro puts on a show of being pleasant and welcoming, but the mask briefly slips when told that Peter, Egon, and Winston aren't visiting the park yet.
- Bittersweet Ending: Karro Zans and his league of ghosts have been trapped, the possessed characters have been exorcised, and Egon has recovered from his cold... but the rest of the Ghostbusters and Slimer have caught his cold.
- Covered in Gunge: Slimer ends up getting ectoplasm all over Mrs. Spengler's front.
- Demonic Possession: Ray, Janine, and Slimer end up being possessed by ghosts at Ghostworld, then later the demon in charge summons ghosts to possess Peter and Winston also.
- Did You Just Flip Off Cthulhu?:
- Subverted. Janine addresses what appears to be a demon as "slimehead", but he turns out to be a man in a costume.
- Played straight when Egon calls Karro Zans's (a demon) plan unoriginal to his face and teases him with lines like "Temper, temper", then when he calls the humans and Slimer his servants, Peter replies, "Sorry, it's servant's night off".
- Dramatic Irony: The audience finds out that Ray, Janine, and Slimer are possessed long before Peter and Winston do.
- "Eureka!" Moment: When told the park's owner is Carl Sands, Egon has a pretty good idea which ghost they're dealing with and looks up the entry to confirm it.
- Evil Laugh:
- Sands laughs evilly when he turns into a demon and again when Peter and Winston show up.
- The possessed Ray and Slimer laugh evilly while planning something.
- Evil Sounds Deep: Karro Zans has a very low voice.
- Flaw Exploitation: Karro takes a good deal of pride in his cleverness. Egon plays to that by insulting his plan's lack of originality and his overall intelligence, as well as keeping him off balance with a game of cat and mouse.
- Foul Medicine: Mrs. Spengler's cold remedy (which apparently comes from her husband's side of the family) consists of chicken soup, orange juice, prune juice, and garlic, and Egon hates the taste of it.
- Happy Dance: Slimer dances for joy when watching his paper plane fly and when Ray accepts the invitation.
- Illness Blanket: At one point, the sick Egon can be seen wrapped in a blanket.
- Last-Name Basis: The purple demon addresses the Ghostbusters by last name.
- Last-Second Joke Problem: All the characters who'd been possessed have been exorcised, Egon (who'd had a cold) has recovered, and the ghosts and their demon leader have been caught... however, the final scene reveals that the rest of the Ghostbusters (and Slimer, somehow) have caught Egon's cold.
- Mundane Made Awesome: Mrs. Spengler calls her remedy "new and improved" despite the only change being her new blender.
- My Beloved Smother: Mrs. Spengler shows up to nurse Egon back to health even though he didn't really want her to and she feeds him a nasty-tasting remedy.
- Never Mess with Granny: Mrs. Spengler is quite overbearing, but when she accompanies Egon to the amusement park, she proves an invaluable ally. She's familiar enough with her son's ghost-catching equipment to use it flawlessly and, after freeing Janine, works with the younger woman to free the other possessed Ghostbusters. Mrs. Spengler even joins in the final battle by using the Ecto-1's headlights (which are programmed to hurt the undead) like a professional.
- Not So Stoic: Egon sticks out his tongue and exclaims, "Yuck!" when he drinks his mother's medicine. Later, after Zans is defeated, he joins in when everyone cheers and punches the air.
- Out Sick: Egon doesn't go to Ghostworld or to attack the fish ghost because he has a cold, though he does still show up to save his coworkers and Slimer.
- Plague Episode: Downplayed. For most of the episode, Egon is the only one sick, but at the end the rest of the Ghostbusters and Slimer catch his cold.
- Priceless Ming Vase: Subverted. Slimer's paper airplane knocks over a vase, but he catches it before it can break.
- Properly Paranoid: After seeing the commercial featured the possessed Ghostbusters, Egon insists on going to the park and expresses certainty that something bad is going down. When Janine is later freed and explains the situation, Egon says this is exactly what he was worried about.
- Pun: Winston describes the fish ghost as "fishy".
- Ray Gun: The "ghost boppers" are handheld weapons that fire blasts of energy.
- Red Eyes, Take Warning: When Janine and Slimer are possessed, their eyes turn red.
- Sneeze Interruption: The first sign something is wrong with Egon is when he sneezes in the middle of a sentence:Egon: "It sure is good to be (sneezes) home."
- Soup Is Medicine: Chicken soup is one of the ingredients of Mrs. Spengler's cold remedy.
- Speak in Unison:
- When Peter and Winston try to escape, they yell, "We're outta here!" in unison.
- The possessed Ghostbusters speak all at once on TV.
- Tempting Fate: Janine tells Slimer his paper plane will never fly. It does.
- Transformation of the Possessed:
- Downplayed. When Ray, Janine, and Slimer get possessed they mainly look normal, except their eyes have changed colour and Ray and Janine have grown fangs.
- Played straight when the possessed Janine then shapeshifts into a monster.
- Villainous Breakdown: Karro is quite the smug villain, but Egon putting one over on him and mocking him in the process quickly take a toll. By the time they're face-to-face again, Karro is done with any pretense of appearing calm and collected.Karro: PREPARE TO DIE, SPENGLER!
- Voluntary Shapeshifting: Karro Zans can turn in between human and demon forms.
- We Need a Distraction: Egon does whatever he can to keep Karro busy, buying Mrs. Spengler and Janine time to free the others.
