The Ghostbusters are fighting off a demon and Egon (on Ray's command) tries using a weapon called a "destabiliser", but the demon deflects it and Egon ends up getting zapped instead and vanishes into thin air. The rest of the Ghostbusters trap the demon in a ghost trap, then return to the firehouse.
Janine asks where Egon is, and they explain what happened (revealing that the demon's name is Arzun and he comes from a place called the Netherworld). Ray blames himself, but then a voice that sounds like Egon says, "No need for self-recrimination". Egon appears, but he's noncorporeal as the result of the accident. He explains that he's still alive, but for all intents and purposes, he's a ghost, and he feels himself continuing to destabilise, believing he'll fade into oblivion within 24 hours.
Ray determines that they can build a machine to save Egon, but it's too expensive. They go to a factory to catch a ghost, and Egon reveals he can sense where ghosts are. Peter ends up accidentally riding on the assembly line, while Winston and Ray end up in a pool of pink liquid. Egon says, "Boo!", scaring the ghost, who flies towards Winston and Ray. Ray gets up and traps the ghost, then Peter climbs off the assembly line. Meanwhile, Egon is in pain, so to make money, the Ghostbusters catch some more ghosts, which is set to a montage.
That evening, Ray builds the device to get Egon back, but he vanishes. Ray uses his computer to determine that Egon is alive but he's in the Netherworld. This is bad news, since only ghosts can enter the Netherworld and only demons can leave. Janine demands that Ray bring Egon back, and he reveals that there is one way, but it's dangerous. Peter and Winston convince him to do it, since in Winston's words, "It's for Egon, Ray!". They put on some watch-like devices that will allow them to travel to the Netherworld for an hour and only return if there's four of them.
They go to the Netherworld and track Egon with a PKE meter. They find a blue terror dog which chases them, but Peter finds out that the proton packs work in the Netherworld when he accidentally zaps a hole in the ground, so he uses the proton pack to zap the terror dog. Unfortunately, the PKE meter breaks, but then an old man from the past approaches and tells them not to attack. He reveals himself to be an alchemist named Heironymus, who thinks the Ghostbusters are wizards. Peter says that they are looking for another "wizard" and Heironymus says that he's imprisoned by Arzun's brother. Peter asks Heironymus to lead them to the place, and Heironymus agrees, as long as they do him a favor in return.
Heironymus leads the Ghostbusters to a dungeon, revealing that he was brought to the Netherworld against his will by demons. The rest of the Ghostbusters find Egon, blast through his prison, and ask Heironymus to name his price. He asks to be returned to Earth, but in order to do that, they'll need to leave one of them behind. Demons start chasing the five of them, and while Winston and Peter fight them off with proton packs, Ray volunteers to stay behind. Egon wants to stay instead since he feels it's his fault, then Peter volunteers to stay instead since "Neither of you would last five minutes here", but Winston refuses to leave Peter behind. Heironymus, however, changes his mind about going back to Earth since it's changed too much while he's been gone. Winston gives Heironymus a proton pack to deal with the demons, then the Ghostbusters go back to the firehouse.
This episode provides examples of:
- Comically Missing the Point: When Egon is revealed to be in the Netherworld, Janine thinks he is in the Netherlands, i.e. Holland.
- Continuity Nod: Winston mentions that he hasn't seen a terror dog in a long time. The last time he saw one was in Ghostbusters (1984).
- Disney Death: When Egon disappears after Ray uses the restabiliser on him, it seems as though he died (especially since he said he was "losing it"), but it turned out he survived but was transported to the Netherworld.
- Fish out of Temporal Water: Defied when Heironymus decides not to return to Earth because the environment would be too unfamiliar after so much time has passed.
- Godzilla Threshold: The Ghostbusters use Ray's method of travelling to the Netherworld despite it being very dangerous since if they don't use it, Egon will be confined to the Netherworld for the rest of his life.
- Immortality: It's implied that no one can die in the Netherworld, since Herionymus talks like he's from the past.
- Living Ghost: An accident results in Egon becoming noncorporeal and able to sense ghosts. However, he must be turned back into a normal human because being noncorporeal too long would make him disappear completely.
- Loud Gulp: Ray audibly gulps when Peter and Winston demand that he use his dangerous method of saving Egon.
- My God, What Have I Done?: Ray blames himself for Egon's "death", since he was the one who told him to use the destabiliser.
- Nice Kitty...: Ray nervously says, "Nice doggy" to the terror dog in hopes that it doesn't kill him.
- Psychic Powers: Egon, while a "ghost", can sense where ghosts are.
- Ray Gun: The destabiliser is essentially a gun that shoots blue energy.
- Say My Name:
- Ray shouts, "Egon!" when Egon is zapped.
- Janine says, "Egon?! Egon!" when Egon groans in pain.
- You Are in Command Now: With Egon at first incapacitated and then gone, Ray's the one who steps up and leads the team.
