In Knock, Knock, subway workmen accidentally open a door that unleashes all of the evil from hell and it begins to spread across New York, primarily through the subway tunnels. This time, the Ghostbusters may not come back alive! 
The Ghostbusters are on a night bust at the bowling alley, Bowl Rama Bowling. The problem is that the bowling balls, pins, shoes, and tacky bowling shirts are under the influence of ghosts. After utterly wrecking the place, the Ghostbusters emerge victorious and tired. They head back to the Firehouse where they find another mess; food is all over the floor and Slimer is eating on the file cabinet. Janine tried to stop Slimer, but failed. Peter comes up with a solution, he turns on his Proton Pack and aims for the Spud. The other Ghostbusters tackle him before he could shoot. With everyone mad, they all argue about whose turn it is to empty the ghost trap from the bowling alley.
Subway workers find a door in a tunnel that growled, "Do not open until doomsday!" The foreman responded, "We are not going to stop 'cause some nut door says so!" The door responded by opening a portal. The subway workers suffered a horrible fate before they realized what they'd done. A swarm of ghosts appear and follow the subway tracks to points unknown while others went above ground.
Our tired heroes are slipping into their pajamas when Janine hits the alarm. The MTA called and said something was wrong with the New York City Subway. The Ghostbusters suit up and take off. When Ghostbusters arrived at a subway station, they find that every square inch is covered in Ectoplasm. Even worse, all the graffiti has sprung to life. They board a nearby train to escape, but the train was possessed by a powerful force. After the worst subway ride in history, the guys get off at a station. Then, the train they arrived in deflates like a balloon.
According to the P.K.E. Meter, the center of the disturbance was 10 miles down the line and it is sending out massive amounts of poltergeist energy that changes everything. Winston trips over a stone tablet, buried for ages and written in Sumerian. They come upon a doorway into the Nether-Region and learn after Doomsday ended the world, the Door was supposed to replace it. They enter the place of lost souls one mile in and finally to the Doomsday Door. The flow from the Door is used to supercharge the Proton Packs. They switch to full dispersion mode, wide angle and shoot into the core. The Proton Streams exit the core and hits the ghosts and energy. The Ghostbusters switch to Capture Mode and pulls it all back into the Doomsday Door. They begin to feel the pull and fire to force themselves out the door just in time. The Doomsday Door closes and states its warning once more.
At the Firehouse, Peter is angered again when he finds Slimer inside the refrigerator. The others hold him and talk him down. Once everyone is asleep, Peter sneaks away and feeds Slimer then wishes him goodnight.
|Doomsday Door: Do not open until doomsday.|
|Peter: Alright. Alright. But if I don't come out of this alive, I'm gonna be real upset.|
Peter: "Slimer!" (whips out his Particle Thrower)
Ray: "PETER WAIT!" (he and Winston tackle Peter and restrain him)
Peter: "JUST LET ME BLAST HIM! ONCE! IS THAT REALLY TOO MUCH TO ASK!?"
- "Knock, Knock" is sort of the gateway episode from season 1 to season 2 as it appears to have been animated for season 1.
- The Doomsday Door had Sumerian writing on it and was supposed to be used after the world was destroyed.
- Gozer was a Sumerian god who was supposed to destroy the world. Both manifested in New York City.
- Ray Stantz has season tickets to the New York Mets.
- A sewer cover reads "AJIA-DO," the name of the Japanese animation company.
- Two versions of this episode exist. When the Ghostbusters are on the train, one version has series music, while the other has "The Boogieman" (performed by Tahiti—from the soundtrack album).
- The Tahiti Music Version can be found on the Magic Window home videocassette released in the 1980s.
- Multimedia samples can be found on Spook Central's Alternate Episodes page.
- The first draft of the script, submitted on February 26, 1986, differs from the episode. For example:
- In the first draft, the Skeleton Woman is the first entity the Ghostbusters encounter just as they arrive at the subway station. 
- In the first draft, Egon admits he can't read Sumerian (or ancient Egyptian, Greek, Indian and Swedish). 
- In the first draft, the language on the Doomsday Stone changes from Sumerian to ancient Egyptian to Greek to Indian to Swedish.
- In the first draft, the Skeletal Tour Guide and Ghostbusters have a brief standoff. Peter aims his thrower at the Tour Guide. The Tour Guide, in turns, puts his hands on his hips and looks very indignant. Peter decides not to shoot. 
- In the first draft, during the news segment, the second news anchor quotes the Mayor. 
- In the first draft, while contemplating the nature of the Place of Lost Souls, Egon mentions the Containment Unit is based on gravitetic principles similar to a black hole. 
- In the first draft, Ray has season tickets to the Yankees. 
- In the first draft, before instructing everyone to go to Full Dispersion Mode, Egon has everyone activate a feature on their Proton Packs, that is deleted from this episode and the series: Auxiliary Power Receptors. They are small, dish-like antenna that swivels out of the packs and collect energy from the waves swirling around. 
- In the first draft, as the Ghostbusters pull back all the ghosts and energy back through the Doomsday Door, the Skeletal Tour Guide makes a second appearance clinging to the boat. 
- ↑ Eatock, James & Mangels, Andy (2008). The Real Ghostbusters Complete Collection booklet, p. 6. CPT Holdings, Inc.
- ↑ Straczynski, J. Michael (2009). The Real Ghostbusters Complete Collection Volume One Disc Five, "Knock, Knock" Script p. 12. CPT Holdings, Inc. Line reads: "Heading toward a figure in the mist: from behind, it seems to be a very attractive, well-dressed blonde woman."
- ↑ Straczynski, J. Michael (2009). The Real Ghostbusters Complete Collection Volume One Disc Five, "Knock, Knock" Script p. 17. CPT Holdings, Inc. Egon says: "It's Sumerian. I can't read Sumerian. No, now it's--it's ancient Egyptian! But I can't read that, either. Greek! No, can't read that, either. Indian? No. Swedish! No, that's out."
- ↑ Straczynski, J. Michael (2009). The Real Ghostbusters Complete Collection Volume One Disc Five, "Knock, Knock" Script p. 22. CPT Holdings, Inc. Line reads: "Then it turns, sees the Ghostbusters. Venkman raises his particle-beam thrower, more as introduction than threat. ON TOUR GUIDE who stands, skeletal hands on skeletal hips. He looks very indignant."
- ↑ Straczynski, J. Michael (2009). The Real Ghostbusters Complete Collection Volume One Disc Five, "Knock, Knock" Script p. 25. CPT Holdings, Inc. News Anchor #2 says: "Says the Mayor of New York, "Frankly, it's a mess.""
- ↑ Straczynski, J. Michael (2009). The Real Ghostbusters Complete Collection Volume One Disc Five, "Knock, Knock" Script p. 27. CPT Holdings, Inc. Egon says: "The ecto-containment unit where we store the captured ghosts is based on gravitetic principles similar to a black hole."
- ↑ Straczynski, J. Michael (2009). The Real Ghostbusters Complete Collection Volume One Disc Five, "Knock, Knock" Script p. 31. CPT Holdings, Inc. Line reads: "But--I just got season tickets to the Yankees! The best seat I've ever had!"
- ↑ Straczynski, J. Michael (2009). The Real Ghostbusters Complete Collection Volume One Disc Five, "Knock, Knock" Script p. 33-34. CPT Holdings, Inc. Line reads: "ON BACKPACKS As the Ghostbusters each push a button. Out of the back of each of the devices a panel swivels around, revealing a small, dish-like antenna. ENERGY WAVES floating through the surrounding ether are caught up on the receptors. And the power indicators on the back of their packs glow brightly, moving further and further up. POWER HUM."
- ↑ Straczynski, J. Michael (2009). The Real Ghostbusters Complete Collection Volume One Disc Five, "Knock, Knock" Script p. 36. CPT Holdings, Inc. Line reads: "As the TOUR GUIDE, clinging to the boat of skeletal tourists, come SHOOTING THROUGH FRAME, through the doorway ."
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