NOTE: This spoiler was submitted by Jeremy

New York City, 1904 – Members of the New York Fire Department arrive at The Manhattan Adventurers Society in response to smoke billowing out of a room. The smoke is sucked back into the door frame as one of the firefighters grabs the doorknob. His hand begins to freeze, and the firefighters break the door down revealing a room filled with dozens of frozen corpses while an unknown language plays from a phonograph. In the shadows, a mysterious figure holds an orb containing an ancient evil, and once the orb opens, the bodies shatter, and a pair of eyes stare out from within.

Present Day – The Spenglers – Callie (Carrie Coon), Phoebe (Mckenna Grace) and Trevor (Finn Wolfhard) – plus Mr. Grooberson (Paul Rudd) have moved to Manhattan from Summerville to become full-time Ghostbusters. They are chasing a ghost called the Sewer Dragon through the streets. Trevor nearly traps it before it starts to fly out of range, but Callie sets out a drone trap, which is able to nab the ghost, but Grooberson ends up crashing into a rack of city bikes.

The family meets with the Ghostbusters’ old nemesis, Walter Peck (William Atherton), who has now become the Mayor of New York City. He chastises them for not only continuing their work and causing damage to the city, but also for employing Phoebe, as she is still a minor. The incident also hits the news, putting a negative look on the Ghostbusters once again. It’s also shown that while Grooberson is trying to make an effort as a surrogate father figure to Phoebe and Trevor now that he’s dating Callie, Phoebe still hasn’t quite accepted him, but Callie does her best to support him.

The Ghostbusters return to their headquarters, which is being bankrolled by Winston (Ernie Hudson). They are visited by Janine (Annie Potts) just as they are adding the Sewer Dragon to their containment unit, which is almost a struggle since the containment unit has reached maximum capacity. Meanwhile, Trevor spots green slime from upstairs and goes to find a mountain of junk food. He then meets Slimer, who flies through Trevor and covers him in goo before he disappears.

Ray (Dan Aykroyd) and Podcast (Logan Kim) are now trying to use artifacts from other customers to try and forge a connection to the afterlife for their new online show. A woman uses a watch from her late husband for Ray to try and read, but he gets nothing and Podcast just smashes the watch with a hammer. They are visited by Nadeem Razmaadi (Kumail Nanjiani), who brings them the same orb seen in the beginning. Ray tries to use his PKE meter on it, but the energy emitted from the orb causes a surge that breaks the meter.

Phoebe plays chess by herself in the park, when one of the chess pieces start moving on their own. She is met by a ghost girl named Melody (Emily Alyn Lind), who died in a tenant fire and always appears with small flames. She tells Phoebe that she isn’t very confident about her chances of crossing over. After chatting with Phoebe, Melody is later seen out on the streets on her own, speaking to a growling voice and asking if Phoebe is really the one he wants for his plan.

The family arrives at a ghost research facility that Winston has also funded, with parabiologist Lars Pinfield (James Acaster) having been hired. Lucky (Celeste O’Connor) is working there as well, and she reunites with Trevor. They bring the orb, which Lars notes that the other captured ghosts in there have become drawn to. Later, the facility experiences a power surge when Lars tries to extract the energy source from the orb, and when Lars goes to grab the orb, his arm is nearly frozen off.

The Ghostbusters get a call, so Phoebe and Podcast go to investigate. They arrive at a diner, where they find Melody is the ghost that is bothering people. Podcast gets ready to have her trapped, but Phoebe cannot bring herself to do it.

The team visits Nadeem at his shop for further information on the orb. A PKE meter indicates that Nadeem himself has spiritual energy. Lars and Lucky bring Nadeem to Venkman (Bill Murray) for further research, and it is discovered that Nadeem has the ability to manipulate fire whenever Venkman starts to get him angry.

Melody visits the Firehouse and continues chatting with Phoebe, who shows her the facility and the containment unit. She also takes interest in the orb and notes a strange language coming from it.

Phoebe and Podcast join Ray in going to the New York Public Library to learn more about the orb’s origins. They meet an expert named Dr. Hubert Wartzki (Patton Oswalt), who explains it for them. Centuries earlier, a power-mad king sought help from a powerful entity called Garraka in his conquest, but Garraka had his own plans to use his powers to bring forth a new ice age. A team known as the Firemasters (Nadeem’s ancestors) used their powers to imprison Garraka in the orb and steal his horns that gave him his power. If Garraka becomes free once more, New York would experience “the death chill” built from their fear. The orb then starts to emit the same language heard earlier.

As the team starts leaving, a ghost that stowed away with Podcast goes loose around the library, forcing the three to chase it. Ray even runs into the original Library Ghost (from the very first movie) at one point and is still spooked by her. The ghost then makes its way outside and possesses the lion statue Fortitude outside the library. Phoebe uses her proton pack to fire a stream and destroy it, just before the cops arrive. Peck uses this as another good reason to come down on the Ghostbusters and try to get them shut down for good. He has the Firehouse condemned and has their equipment repossessed. Phoebe is also arrested for refusing to comply with Peck, while Winston chastises Ray for getting her and Podcast in trouble.

Melody visits Phoebe again as she wants to be alone. After they talk some more, Phoebe decides to use a way for her and Melody to be on the same physical plane, by having Phoebe briefly separate her spirit from her body for two minutes. The procedure works, and Phoebe becomes a ghost, but this is when Melody reveals her true intentions. She is working for Garraka, who cannot use a ghost like her to open the orb, so he needed a living person (Phoebe) to do the job for him, in exchange for helping Melody cross over to be with her family. He possesses Phoebe’s body to speak the incantation, and the demon ghost comes back to life.

Phoebe’s spirit returns to her body as the rest of the team tries to help her. Lucky tries to fire a proton blast at Garraka, but he freezes and shatters the stream. Callie and Grooberson help Phoebe while Ray, Winston, and Janine also arrive. They realize Garraka is going to try and free the other ghosts for his death chill plan to take effect. Outside in the city, everything starts to freeze over and produce hundreds of deadly icicles that nearly kill people.

Everyone makes it to the research lab to get equipment before Venkman and Nadeem join in the fight. Ray tries to get Nadeem to find a way to tap into his Firemaster abilities to help fight Garraka. Meanwhile, the possessor ghost comes back and tries to kill everyone by possessing a proton pack, but Nadeem is able to redirect the blast out the window with his powers. The ghost then possesses pizza and is promptly devoured by Slimer.

Lucky tells Phoebe that regular proton blasts won’t be enough against Garraka, so Phoebe lines her blaster with brass since it is similar to the orb’s material that can trap Garraka. The demon ghost makes his way to the facility, and the Ghostbusters’ efforts against him remain futile, and he has almost all of them trapped in ice. Nadeem tries to use his fire powers on Garraka, but he has run out of lighter fluid since he was practicing with it a lot. Downstairs, Phoebe talks to Melody, who is remorseful of her actions, but Phoebe tells her that Garraka was never going to help her cross over like he said, and that she had the ability to do that herself all along.

Phoebe goes to face Garraka herself, with her brass actually managing to strike Garraka, but he still manages to overpower her. Melody then uses her flame to light a match for Nadeem, allowing him to use his powers to bend fire against Garraka. The Ghostbusters are freed, but Garraka breaks open the containment unit, unleashing every ghost the team has ever captured. However, this leads the old Ghostbusters to realize that Garraka has just made an even bigger containment unit, and with everyone’s combined efforts, they can stop him. The Ghostbusters fire their proton blasts alongside Nadeem’s firebending, and Garraka is quickly brought down and de-horned once again as he becomes trapped. The ice outside begins to disappear, and Melody finally crosses over after saying goodbye to Phoebe.

The Ghostbusters emerge to find a crowd of cheering New Yorkers…and Peck. He gets ready to shut them down yet again, until the other citizens force him to support them and view them as heroes since, if not for them, the city would be under three feet of ice. Peck has no choice but to change his tune. Moments later, Slimer and the Sewer Dragon fly out of the facility and into the city. The Spenglers and Grooberson then suit up (with Phoebe calling Grooberson “dad”) and head out to bust more ghosts.

During the credits, the mini Stay Puft Marshmallow Men (from the last movie) are seen stealing a truck from the official Stay Puft company from an unwitting driver.

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The Spenglers - Callie, Phoebe, and Trevor - and Mr. Grooberson move to New York City to become full-time Ghostbusters. However, the original team's nemesis, Walter Peck, now mayor of the city, is still looking for reasons to shut them down, especially with someone as young as Phoebe in their employment. Meanwhile, Ray Stanz is given a mysterious orb by a man named Nadeem Razmaadi. The orb contains an ancient evil ghost named Garraka, who, if set free, can bring about a new ice age.

Phoebe befriends a ghost girl named Melody, unaware that she is working for Garraka in exchange for his help in crossing over to be with her family. When Phoebe separates her spirit from her body to be on the same physical plane as Melody, she reveals her true motives by having Garraka possess Phoebe's body since only a human can read the incantation necessary to free him. Garraka plans to unleash all the ghosts captured by the team since 1984 to bring about "the death chill", produced by the fear of everyone in the city. Extreme ice and cold begins to overtake the city as Garraka goes free.

With Peck having shut down the Firehouse due to an earlier incident, the Ghostbusters go to a research facility bankrolled by Winston Zeddemore where they have spare equipment and more containment units. Nadeem joins the fight because he comes from a line of fighters called Firemasters, and his ancestors were the ones who previously imprisoned Garraka inside the orb. The demon ghost makes his way to the facility and breaks open the containment unit. However, Ray figures that the facility can be retrofitted into an even bigger trap. The old team (Ray, Winston, Peter Venkman, and Janine Melnitz) and the new (Phoebe, Trevor, Lucky, Podcast, Callie, Grooberson, and a new guy named Lars) team up with Nadeem and a remorseful Melody to utilize both the proton energy and the firebending powers to trap Garraka once and for all. Melody also crosses over after saying goodbye to Phoebe.

The Ghostbusters are once again seen as heroes, and Peck has no choice but to voice his support for them. Slimer and a sewer ghost escape, leading the Spenglers and Grooberson to go after them as a family.