A man drops a spell book off at the library for safekeeping, and that night, a tentacled beast appears in said library and scares a guard into unconsciousness. The next morning, Ray reads about the book in the library, revealing that it's the Necronomicon. Then, the Ghostbusters get called to the library.
At the library, the Ghostbusters discover the Necronomicon was stolen, and Egon detects high paranormal readings. Outside, the Ghostbusters track the reading, and Egon explains that the spells in the Necronomicon open portals to dimensions full of dangerous beasts, such as Cthulhu, who's even worse than Gozer.
They go down to the sewer and find it full of nasties, which they fire at, but they can't trap, so they run away. Back at the firehouse, Egon and Ray explain that the beasts seem to be Cthulhu's minions, and due to how the stars are aligned, if they don't get the book back by that night, cultists may wake Cthulhu up.
The Ghostbusters fly to a castle and meat another occult scientist named Alice Derleth who Peter crushes on. The Ghostbusters and Alice go to a bookshop and meet the cult, and they summon another of Cthulhu's brethren, which Alice banishes with a spell.
The Ghostbusters and Alice leave to find cult magazines that might teach them how to defeat Cthulhu. That night, they drive off and track Cthulhu, who the cult wakes up. The Ghostbusters fire at Cthulhu, but it's immune to proton streams, so Alice tries a spell, but it doesn't work either. Ray reads the magazine, which suggests zapping Cthulhu with electricity.
Peter baits Cthulhu to a rollercoaster, where they zap it, and then it gets hit by lightning. This defeats him, and the cultists get arrested. Peter then wants to date Alice, but changes his mind upon finding she's a nerd.
This episode provides examples of
- Absurdly Spacious Sewer: The sewer has plenty of room to walk around.
- Achilles' Heel: As in the original story, Cthulhu reacts badly to electricity.
- Admiring the Abomination: When Egon reveals that there's an evil entity at the library, Ray says earnestly, "Great!".
- Big Damn Heroes: After Cthulhu is defeated, the cultists close in on the Ghostbusters and Alice, only for the police to arrive and detain them.
- Brainless Beauty: Discussed when Peter's way of calling Alice pretty is saying she doesn't look smart.
- Cast from Stamina: Casting the spell makes Alice woozy.
- Compliment Backfire: Peter thinks smart women are ugly, so when he meets Alice, he tries to call her pretty by saying she doesn't look smart, which offends her.
- Death Glare: The second time the guys try blasting him, Cthulhu looks down right at them.
- Did You Just Flip Off Cthulhu?: Almost literally — Peter says to Cthulhu itself, "Okay, squidface, I'm gonna make calamari out of you!".
- Did You Just Punch Out Cthulhu?: An example involving the actual Cthulhu, when the Ghostbusters actually manage to disable Cthulhu with lightning.
- Dramatic Thunder: It thunders as Cthulhu is summoned for the second time.
- Dramatic Unmask: Winston does this to the head cultist, revealing him to be Clark Ashton.
- Faint in Shock: The guard passes out when he sees Cthulhu.
- Green and Mean: Cthulhu's evil minions are all dark green.
- Hope Spot: After the proton packs prove ineffective against Cthulhu, Alice attempts a spell again. He is briefly surrounded by energy and convulsing, but he soon shakes it off.
- Leaning on the Fourth Wall: Peter is confident that he and the others will eventually get the Necronomicon and that the next time the stars will align so that Cthulhu will be resurrected is that night.
- Losing a Shoe in the Struggle: Peter's boot gets pulled off during the sewer fight.
- Mythology Gag:
- Egon says that Cthulhu makes Gozer (the main antagonist of the first movie) look like Little Mary Sunshine.
- Peter tells a man to "back off, man, I'm a Ghostbuster", referencing his line "Back off, man, I'm a scientist" from the first movie.
- No-Sell: Cthulhu is immune to proton streams.
- Oh, Crap!: When the proton streams don't work on Cthulhu, Winston says, "We're in serious trouble!".
- One-Liner: During the scuffle with the sewer beasts, Peter says, "These guys sure can't take a hint".
- Parental Bonus: The episode is filled with references to the Cthulhu Mythos, such as guest characters being named after Lovecraft's collaborators, name-drops of various characters and objects from assorted stories, and a visit to Miskatonic University.
- Rip Van Winkle: Cthulhu has been sleeping for centuries, and the spell wakes it up.
- Science Hero: In addition to Peter, Ray, and Egon, Alice is also a paranormal scientist and she helps defeat Cthulhu.
- Shout-Out:
- Peter thinks Cthulhu "looks like Godzilla wearing an octopus hat".
- Writer Michael Reaves lifted the words to Alice's stone spell from one of his scripts for Dungeons & Dragons (1983).
- According to Peter, Mr. Howard's house would put a scare into Stephen King.
- Shut Up, Hannibal!: After Clark Ashton is restrained by the cops and unmasked, he goes on a rant.Ashton: Fools! You may've won this time, but Cthulhu cannot be destroyed! He waits and dreams in the deep, and the cities of man shall fall before him!
Peter: Yeah, yeah, sure. Book him, boys. - Smarter Than You Look: Discussed when Peter says that he wouldn't expect a pretty woman like Alice to be smart.
- Tempting Fate: Peter expects Alice to be ugly since he thinks smart women are ugly, but when they arrive, she's pretty.
- Tentacled Terror: In addition to Cthulhu itself, its minions also have tentacles.
- Theme Music Power-Up: The theme tune plays as Cthulhu is being defeated.
- This Is Gonna Suck: After Cthulhu gives them the Death Glare.Winston: Uh-oh. I think we made it mad.
Egon: RUN FOR IT! - A True Story in My Universe: Ray suggests researching old pulp magazines to figure out their next move, saying he recalled an old Weird Tales story explaining how to defeat Cthulhu. Alice says Lovecraft and his associates did use the Necronomicon as research for their stories. It ends up being exactly right, with Ray reading the story and citing electricity as the big guy's weakness.
- Tuckerization: A few guest characters are named in part after a few of H.P. Lovecraft's inner circle: Alice Derleth (August Derleth), Clark Ashton (Clark Ashton Smith), and Mr. Howard (Robert E. Howard).
- You Fool!: Clark Ashton's first exclamation, after being unmasked and arrested.
