In Heart of Darkness, ghosts wearing strange collars are behind a series of thefts. After one of Egon's former colleagues is killed in a confrontation, the Ghostbusters must stop them from opening a portal to the Netherworld!
Three ghosts flew to the Bright Wave High Tech Industries facility. They entered a specific room through the ventilation system. One ghost spotted something and pointed to it, a mounted gun of some sort. Another ghost removed the gun from the mount but knocked over a cart. Two guards heard the noise and checked the room. They noticed the LB-7, the gun, was missing. One guard was about to call the police when a ghost shoved him into a wall. Another ghost tossed the second guard into the wall, too. The ghosts left the facility with the LB-7.
Some time later, Bright Wave was visited by the Ghostbusters, police, and an ambulance. Roland spoke to a scientist about the LB-7. It was the company's most sophisticated prototype laser. Eduardo swept the room with his P.K.E. Meter and confirmed paranormal activity. He joked around about tenting the place but the scientist didn't catch on. Roland jotted down some notes. The scientist turned and was frightened by Slimer. Slimer smelled the LB-7's mount. Roland calmed down the scientist and told him Slimer worked with them. Roland convinced him to go wait in his office and he would get a full report from them. Slimer sneezed then hovered around Eduardo, getting slime on one of his shoulders. Eduardo pushed him away and Slimer bounced off Roland's gut. Eduardo asked what was the reason Slimer was with them on the call. Roland revealed he was training Slimer to track down other ghosts as a fail safe for when the P.K.E. Meters ever malfunctioned. He picked up the idea from watching his brother-in-law train his bloodhound. Eduardo found it an insult to dogs. Eduardo returned to the discussion at hand and asked what ghosts would want with a laser. Roland had no idea, as well.
Elsewhere in the city, Kylie and Garrett attended a Paranormal Exhibit hosting antiquities from Russia. Garrett wasn't too impressed. Kylie recited the quote of 'the magic of yesterday is often the science of tomorrow,' but Garrett didn't catch on. Kylie saw the Toltec Skull on display. Garrett still wasn't impressed that was the reason he was dragged to the exhibit. Kylie replied she took him along so she wouldn't ride the subway alone. Garrett frowned. Suddenly, the same three ghosts from the Bright Tech theft arrived. Garrett was not happy to see ghosts during off duty hours. Two ghosts lifted up Garrett on his wheelchair and flipped him off. The third ghost stole the Toltec Skull. Kylie jumped on its back but the ghost dropped her off on the UFO display. Kylie slid off and struck the ancient electrical battery exhibit. Garrett picked up the battery and tossed into the third ghost's gut. The ghost became agitated but its collar hummed. It and the other ghosts stopped their assault and left. Kylie jumped out the window and watched as the ghosts vanished from her view. Behind Kylie, a shrouded man pressed a remote control. Kylie turned and the man was gone.
At the Firehouse, Kylie and Garrett explained their encounter to Egon and Janine up in the lab. Egon informed them the Toltec Skull was only one of three in existence and apparently held the power to open a door to the Netherworld. Janine asked what ghosts would need that for. Kylie guessed the ghosts were stuck on the physical plane and needed a way back home. Garrett wondered about the collars the ghosts wore. Eduardo, Roland, and Slimer returned. Eduardo described the collars to Kylie's surprise. Roland revealed the Bright Wave guards told them the thieves wore collars. Garrett surmised the other two skulls were in danger. Egon revealed one was in New York City, owned by the millionaire Wilhem Rykhart, and the other in New Jersey, owned by Edward Kirilian. Garrett recognized Rykhart's name. Slimer salivated as Egon put away a decaying sample of Salmonella spores into his cabinet. Janine recognized Kirilian as an old colleague of Egon's. Egon somewhat admitted they worked in related fields. The team split up to make sure the skulls were safe.
Roland, Eduardo, and Slimer took the Mustang to Rykhart's while Egon, Kylie, and Garrett went to Kirilian's place in New Jersey. Roland ordered Slimer to keep an eye on the car. Eduardo wasn't impressed and hit the doorbell. Ecto-1 arrived at Kirilian's but the place looked incredibly worn down. Rykhart assured Roland and Eduardo his Toltec Skull was safe in a vault lined with solid titanium three inches thick. He was amused when he learned the culprits were ghosts. Suddenly, they heard something. The vault door opened and the three ghosts flew out with the skull. Roland and Eduardo followed them upstairs. The butler lounged out of the ghosts' way just in time. Roland and Eduardo opened fire but struck a 300 year old Ming vase instead. Rykhart screamed. The ghosts left through an open balcony door. As Rykhart assessed his destroyed vase, Eduardo advised him to use his money to buy new stuff. Just as Garrett rang the doorbell, Roland hailed Kylie on her radio to warn them about the ghosts. Garrett knocked the door down. They heard Kirilian struggling with someone upstairs. Egon ran up and saw Kirilian grabbed by a ghost. An explosion rocked the house and a fire started. The house started to cave in. Debris blocked the front door so Garrett made a new exit with his thrower. Egon, Kylie, and Garrett watched as the house collapsed. As firefighters put out the remains of the fire, Roland asked about Egon.
Kylie checked on Egon. He revealed he and Kirilian worked together at "The University" and shared some of the same passions. Egon even respected Kirilian in the beginning until his theories became increasingly dangerous. Kirilian proposed controlling ghosts and using them as slave labor. They had a falling out. Soon after that, Kirilian lost his job and they never saw nor spoke to each other again. Egon regretted not calling him. Kylie empathized and talked about her Grandma Rose. Garrett and the others came over wondering what to do next. Egon wanted to avenge Kirilian's death. Eduardo got a reading but couldn't tell which way the ghosts went. Roland asked Slimer to help but he protested. Egon translated it as Slimer refusing to be a slave. Roland reminded Slimer he was an employee, not a slave. Slimer agreed and picked up a trail. Eduardo didn't like the idea of using Slimer. Slimer was predictably distracted by food. He raced over to a Hot Pretzels stand and gorged. Egon picked up high ecto activity. The team ended up at a closed and abandoned junior high school. Kylie recognized it as the one she attended. Garrett opened the doors and they headed inside. Eduardo couldn't resist and asked Kylie what she was like in junior high. Kylie boldly declared the same as she was now, trouble. Egon traced the readings a floor below. They descended the stairs and went down a hall. Kylie became nervous and believed it wasn't the right way. Egon assured her there were strong emanations coming from the end of the hallway. Garrett noticed a display case and saw a photo of young Kylie...8th grade Cheerleader of the Year. Garrett and Eduardo laughed at her expense.
Egon found a door marked "Civil Defense." Civil Defense tunnels were built in the 1950s under schools and government buildings as a precaution at the height of nuclear threat during the Cold War. The P.K.E. Meters started dying out. Egon realized the lead-lined tunnels were the reason. Eduardo was ready to call it a day. Ghosts appeared and attacked them. Slimer growled at one but ran for it. Eduardo wasn't impressed. Two knocked Garrett over and another two kidnapped Egon. The Ghostbusters ran after them but ended up lost in the labyrinth of tunnels. The ghosts tossed Egon into a jail cell while someone gloated outside. Some time later, something entered the cell. Egon grabbed it with his blanket but it was only Slimer. Slimer had no idea what happened to the others. The ghosts returned and took Egon away. Slimer peered out from under the blanket and went to find the others. Egon walked past several ghosts lugging equipment around. He was shoved into a room with a machine set up. Egon noticed the three Toltec Skulls on a pedestal of some sort. A man in costume approached him. It was Kirilian. He staged his death to throw Egon off his trail. Egon tried to dissuade Kirilian from breaching the Netherworld but Kirilian was set on his plan. He developed the Collar of Obedience was a way to subjugate ghosts and planned on using them to enslave demons.
Eduardo was sure something was following them. Roland thought he was just being paranoid. Something bumped into Roland and scared him. It was Slimer. Kylie realized Slimer was saying he found Egon. Slimer led them down the right path. Kirilian manned the controls and lectured Egon on how he came to understand lasers were crucial to energizing the skulls and sending out a sub-atomic vibrational pulse to create a cross-dimensional rift. Egon still tried to plead with Kirilian in vain. Kirilian activated the laser. Ghosts attacked the team again. The portal to the Netherworld was opened and several ghosts emerged. The Ghostbusters arrived. The enslaved ghosts stared at the portal in awe allowing Egon to wrestle Kirilian away from the control panel. Egon couldn't gain access so he tore the panel off. The ghosts returned through the portal. The enslaved ones followed, too. Kirilian brought out his remote control but Garrett blasted it into pieces. Kirilian got caught in the pull of the portal but Egon grabbed him. Surprisingly, Kirilian let go. A ghost grabbed Kirilian and took him through to the other side. The portal closed and the machine exploded. Egon was crestfallen that Kirilian let go. Kylie assured him Kirilian let go a long time ago.
|Eduardo: So, Kylie. What were you like in Junior High?|
Kylie: Same as now ... TROUBLE.
|Garrett: Kylie Griffin. Eight grade Cheerleader of the Year! [laughs]|
Eduardo: Trouble, huh? Ooh, yeah. Looks like you were a real rebel.
- Roland mentions he got the idea to train Slimer when he saw his brother-in-law train his bloodhound. 
- Roland mentions the Ghostbusters never encountered paranormal creatures interested in physics of the non-ethereal plane. Technically, that is untrue since Lillith studied the Ghostbusters' equipment schematics to create her Bio Containment Vortex.
- At the paranormal exhibit, Kylie refers to the Iron Curtain, the socio-political-ideological barrier erected by the Soviet Union post World War II. 
- Garrett mentions "Chariot of the Gods,"  a book written in 1968 by Erich von Däniken. It hypothesized that the various technologies and religions of many ancient civilizations were given to them by ancient astronauts who were welcomed on Earth as gods.
- Kylie appears to quote Edward Teller, a Hungarian-born American nuclear physicist, in response to Garrett. 
- When talking about the Toltec culture, their deity is only generally mentioned as the sun god. This god is Tonatiuh, sun god in Toltec, Aztec, and Mayan culture.
- Egon works with decaying samples of Salmonella spores, a bacteria associated with food poisoning.
- Egon worked with Kirilian at New York State University. 
- Kylie talks about the death of her Great Grandma Rose briefly. 
- When Slimer stops at the Hot Pretzels stand, Eduardo sarcastically calls him Lassie. 
- The Ghostbusters visit the junior high school Kylie attended. 
- Kylie Griffin was "Cheerleader of the Year" when she was in the 8th Grade. 
- Annoyed by her past, Kylie quotes a line from Star Wars, "Long ago, in a galaxy far, away." 
- P.K.E. Meters don't work in lead-lined tunnels. 
- Kirilian ends up going through a portal of his own making, much like what happened to Mr. Tummel in The Real Ghostbusters episode "You Can't Take it With You"
- ↑ Roland Jackson (1997). Extreme Ghostbusters- Heart of Darkness (1997) (DVD ts. 02:47-02:50). Sony Pictures Home Entertainment. Roland says: "I got the idea from watching my brother-in-law train his bloodhound."
- ↑ Kylie Griffin (1997). Extreme Ghostbusters- Heart of Darkness (1997) (DVD ts. 03:16-03:20). Sony Pictures Home Entertainment. Kylie says: "This is the first time the Russians ever allowed these objects to travel outside the Iron Curtain."
- ↑ Garrett Miller (1997). Extreme Ghostbusters- Heart of Darkness (1997) (DVD ts. 03:21-03:25). Sony Pictures Home Entertainment. Garrett says: "You don't really buy into this Chariot of the Gods stuff do you?"
- ↑ Kylie Griffin (1997). Extreme Ghostbusters- Heart of Darkness (1997) (DVD ts. 03:26-03:29). Sony Pictures Home Entertainment. Kylie says: "The magic of yesterday is often the science of tomorrow."
- ↑ Egon Spengler (1997). Extreme Ghostbusters- Heart of Darkness (1997) (DVD ts. 09:08-09:13). Sony Pictures Home Entertainment. Egon says: "We worked together at the University. Shared some of the same passions. I had respect for him in the beginning."
- ↑ Kylie Griffin (1997). Extreme Ghostbusters- Heart of Darkness (1997) (DVD ts. 09:38-09:46). Sony Pictures Home Entertainment. Kylie says: "When my Grandma Rose died, I had so many things I still wanted to tell her but I..."
- ↑ Eduardo Rivera (1997). Extreme Ghostbusters- Heart of Darkness (1997) (DVD ts. 10:50-10:52). Sony Pictures Home Entertainment. Eduardo says: "Oh yeah, he's a regular Lassie."
- ↑ Kylie Griffin (1997). Extreme Ghostbusters- Heart of Darkness (1997) (DVD ts. 11:06-11:08). Sony Pictures Home Entertainment. Kylie says: "My old junior high."
- ↑ Garrett Miller (1997). Extreme Ghostbusters- Heart of Darkness (1997) (DVD ts. 11:57-12:02). Sony Pictures Home Entertainment. Garrett says: "Kylie Griffin 8th Grade Cheerleader of the Year!"
- ↑ Kylie Griffin (1997). Extreme Ghostbusters- Heart of Darkness (1997) (DVD ts. 12:10-12:13). Sony Pictures Home Entertainment. Kylie says: "That was long time ago in a galaxy far away!"
- ↑ Egon Spengler (1997). Extreme Ghostbusters- Heart of Darkness (1997) (DVD ts. 12:50-12:57). Sony Pictures Home Entertainment. Egon says: "These are lead-lined tunnels designed to be impervious to nuclear fallout. In much the same way, they're blocking our P.K.E. readings."
Episode Screen Caps
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