Underground construction workers come across and open a door that was not meant to be opened until Doomsday. The Ghostbusters race to close it before the world really does end.
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- The title of this episode is a reference to the form of joke, a Knock Knock joke.
- The reference of Egon being able to speak Sumerian is a nod to the first Ghostbusters movie. Gozer was a Sumerian god.
- A manhole cover in the episode says "Aija-Do", a Japanese animation company.
- This episode utilizes several sound effects from the Star Wars films, including a laser blast and lightsaber activation, deactivation and humming sounds.
After Peter backed away from a pink-haired, skull-faced ghost.
- Peter Venkman: Say, now when was the last time you saw your dentist, hm?
After narrowly saving the world.
- Peter Venkman: That was great. Can we do it again?
- Doomsday Door: Do Not Open Until Doomsday!
- Peter Venkman: It was just an idea.
Referring to the writing on a stone tablet.
- Egon: It's Sumerian.
- Winston: Can you read Sumerian?
- Egon: In my sleep. Underwater with the lights off. Of course I can read Sumerian.
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|Episodes 26 - 30|
|Episodes 31 - 35|
|Episodes 36 - 40|
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|Episodes 61 - 65|