In Once Upon a Slime, not wanting to return his favorite fairy-tale book to the library, Slimer hides it in Egon's latest invention and accidentally turns it on. The next day the fairy tales have come to life, and the fantastical beings learn that Egon's machine was designed to turn ethereal entities into solid matter! 
After Ray finished reading a fairy tale to Slimer, he noitced the book was two days overdue. He told Slimer that it would have to be returned in the morning. Slimer objected and wanted to keep it. Ray insisted they could check it out again and bid him goodnight. Slimer turned the light back on to stick his tongue out at Ray. Meanwhile, Winston and Egon were working on a new machine in the basement workroom. Winston began to realize it was rather late in the evening when Egon handed him a screwdriver instead of a wrench and vice versa. Peter stepped in to check on the happenings. Winston managed to convince Egon to call it a night just so they finished in the morning and ran some tests.
Unhappy, Slimer took the book at around 2 A.M. and tried to locate a place to hide it. The kitchen refrigerator was too full but Slimer helped himself to some grapes. Slimer tried to hide the book under rug near the edge of the Rec Room but it stuck out. Eventually, Slimer settled for the basement as a hiding place. Unfortunately, Slimer hid the book underneath the new machine of Egon's, which was designed to make ethereal entities into solid matter to facilitate their capture. Slimer inadvertently flipped a switch located on the bottom side of the macine. After he left, the machine hummed to life and glowed eerily.
Later that morning, Slimer helped himself to a stack of pancakes for breakfast. He splashed some syrup on Peter's nose but licked it off for him. Ray came into the question concerned about the lost fairy tale book. Slimer pretended not to know what happened to it. Suddenly, Janine got a call. It turned out an enchanted forest manifested in Manhattan. The Ghostbusters went to investigate. They came upon the Gingerbread House from "Hansel and Gretel" and Slimer started to eat pieces of the building. Winston, Egon, and Slimer confirmed the house was in fact edible. The house then came alive and tried to eat the Ghostbusters. The Proton Streams made quick work of the house and the forest but Egon was stumped as how the phenomenon had happened. He took a sample of the gingerbread's smoldering remains for analysis.
As they headed back to Ecto-1, their car was stolen by a red skinned dwarf who told them they could never guess his name. After Peter's nicknames failed to stop the dwarf, Slimer and Ray quickly realized it was Rumplesliltskin, and upon hearing his name, he disappeared. The guys then saw the White Rabbit and Card Soldiers from "Alice in Wonderland." The soldiers trampled right over the guys.
Upon returning to the Firehouse, Little Bo Peep and her sheep walked out. Before Bo Peep left, she apologized about the rugs. Ray noted how it seemed like every fairy tale was coming to life. Slimer realized it was all his fault and quickly grabbed the fairy tale book from the machine and returned it to the library (scaring a librarian in the process). He returned home claiming that he had solved the problem. Just then a giant bean stalk grew into the sky and giant-sized garbage started began raining down on the Firehouse. Slimer explained about the book and the machine. The machine worked so well that it turned the fairy tales into the real thing. Egon deduced what happened and claimed he could reverse the process if he had exactly the same fairy tale book. Egon and Ray went with Slimer to retrieve the book while Winston and Peter took Eco-2 to investigate the top of the beanstalk.
After they collided with a tomato, the duo rode Ecto-2 upward and came upon a castle on a cloud and were immediately taken down by a giant child with a squirt-gun. The child took the guys to his gluttonous father who prepared to eat Peter and Winston. Egon noticed Ecto-2's emergency beacon on his Belt Gizmo and realized time was short. They found the book and raced back to the machine. Egon reversed the wiring and instructed Ray to place the book on top of the machine. Egon activated the machine, which caused the Giants and the castle to dematerialize just before Winston and Peter were eaten. The guys made it back onto Ecto-2 and flew safely home. The garbage and beanstalk also dematerialized. The Ghostbusters, Janine, and Slimer celebrated with a group hug.
That night, Slimer wanted Ray to read him another bedtime story. Ray tells him he's had enough of fairy tales. Slimer handed him a magazine with a centerfold. Ray pondered why the picture in the centerfold could't come to life. Egon, assuming it was a "girly magazine" asked Ray if Slimer was too young for that "sort of thing." Ray turned the centerfold over to reveal that it is a dessert magazine. Egon and Peter were silent then turned out the lights. Ray caught Slimer looking at the magazine and told him to bed, too.
|Janine: Guys, guys! Boy am I glad you're back. There's a total weirdo inside.|
Peter: You're in luck, Janine. We having a special on weirdos today.
- At the start of the episode, Ray finishes reading "The Frog Prince" fairy tale to Slimer.
- Peter mentions the Smurfs. 
- The first solid manifestation is the forest and Gingerbread House from the "Hansel and Gretel" fairy tale.
- Winston refers to the Gingerbread House as the "original Sugar Shack."  Sugar shack is a term for small cabins designed to collect sap from sugar maple trees and boil it into maple syrup.
- The second solid manifestation, who hijacks the Ecto-1, is the dwarf Rumpelstiltskin from the "Rumpelstiltskin" fairy tale.
- In the fairy tale, the protagonist had three days to guess Rumpelstiltskin's name. In this episode, it took three guesses to get his name right.
- The third and fourth solid manifestations are the White Rabbit and the Queen of Hearts' Army from "Alice in Wonderland" novel.
- The fifth solid manifestations are Little Bo Peep and her lost sheep from the "Little Bo Peep" nursery rhyme.
- The final solid manifestations are the Giant Beanstalk and the Giant from the "Jack and the Beanstalk" fairy tale.
- The New York Public Library and lion statues make a brief appearance in this episode.
- This episode shows that Particle Throwers can short out if doused in large amount water. The Giant's Son shot Peter and Ray with his giant squirt gun.
- Winston refers to the Giant as "Mikey" after the 1972 Life cereal commercial starring the character Little Mikey. 
- Peter offers the Giant his entire collection of baseball cards and mentions he's a fan of San Francisco Giants (although Peter may have been trying to get on his good side). 
- This is the second episode of season 3 to have a sexual innuendo (referring to a centerfold in a magazine). The first was in "Baby Spookums," where a DJ insinuates that something in his love-life occurs once every 300 years.
- This marks the only instance where the Belt Gizmo was used in all Ghostbusters material. 
- ↑ Eatock, James & Mangels, Andy (2008). The Real Ghostbusters Complete Collection booklet, p. 26. CPT Holdings, Inc.
- ↑ Peter Venkman (2009).The Real Ghostbusters- "Once Upon a Slime" (1987) (DVD ts. 06:19-06:21). Time Life Entertainment. Peter says: "I feel like a Smurf!"
- ↑ Winston Zeddemore (2009). The Real Ghostbusters - "Once Upon a Slime" (1987) (DVD ts. 07:50-07:52). Time Life Entertainment. Winston says: "Man, this is the original Sugar Shack!"
- ↑ Winston Zeddemore (2009). The Real Ghostbusters - "Once Upon a Slime" (1987) (DVD ts. 17:22-17:24). Time Life Entertainment. Winston says: "I think Mikey likes us."
- ↑ Peter Venkman (2009). The Real Ghostbusters - "Once Upon a Slime" (1987) (DVD ts. 18:47-18:54). Time Life Entertainment. Peter says: "Tell you what big guy, let us go and I'll give you my entire collection of baseball cards. My team's the Giants."
- ↑ Ray Stantz (2009). The Real Ghostbusters - "Once Upon a Slime" (1987) (DVD ts. 17:49-17:51). Time Life Entertainment. Ray says: "Uh oh. Ecto-2's emergency beacon."
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