Within the first 10 min of the movie, there is a Ghostbusting scene with the Spengler family in NY. They are chasing a water dragon ghost in the Ecto 1. Phoebe uses the gunner seat to catch the ghost, causing damage to a building with the proton stream. The ghost is caught. We get a news report about the situation, in which shows the OG crew and the Statue of Liberty walking in GB 2. Peck is the mayor and finds out that Phoebe is 15 and child labor laws is being violated. Phoebe can't be a Ghostbuster anymore, gets depressed and befriends a 16 year old ghost girl named Melody. Meanwhile, Ray Stanz and Podcast has a podcast where people bring in possessed artifacts and get them read on the PKE meter. If no ghost is detected, Podcast smashes the item with a sledgehammer. Nadeem brings in a orb, which contains the big bad of the movie, to Ray's Occult and wants $50 for the orb. Ray scans the orb with the PKE meter and the meter shatters. Also, we find out that the containment unit in the firehouse is overloaded and we get a scene of Winston introducing the new team to the second headquarters where they have built a new containment unit in the original vision of Egon's plan. There is a device that extracts spirits from objects that will be a pivotal part in the movie later. We find out that Nadeem's grandmother was a part of a group of mystics, known as Firestarters that guarded the orb from keeping the big bad from escaping. At one point, Ray, Podcast and Phoebe goes to the library and meet up with Patton Oswald's character and he tells them the origin of the orb and the big bad, who we now know as Garraka. One of the ghosts that escaped the second headquarters' isolation room in an earlier scene possesses a audio sphere and runs out of the library. We see the library ghost from the first movie scare Ray. Ray almost gets attacked by the lion statue, that is possessed by the possessor ghost. Phoebe pulls out a proton gun from Ecto 5, a motorbike and blows up the statue. They get arrested and Peck has the Firehouse condemned and the proton packs confiscated for destruction. Phoebe goes and vents to the ghost girl, takes her to the second headquarters, gets into the separation machine and becomes a ghost for 2 minutes on some Lydia Deetz thinking. We find out that Garraka can mind control spirits and the ghost girl uses Phoebe's spirit to release him via a ancient chant, since only a human voice can release him. Lucky comes in and tries to blast Garraka, but he freezes the beam and escapes. He goes to look for the Firemaster and says "Are you the Firemaster?" in a callback to the first movie with the keymaster. He kills a convenience worker via ice because the guy was working in the store which was named Firemaster. Nadeem is the Firemaster in which we find out in a earlier scene when Venkman does a test on him while Nadeem has the Louis Tully hat on from the first movie. Everyone suits up, including Annie Potts' fine ass and tries to stop Garraka from reaching the containment unit and releasing all of the ghosts to create a army. Winston had new proton packs made, which was convenient. Phoebe quickly reconfigure her pack that she had hidden upstairs under her bed to affectively hurt Garraka. The containment unit breaks and we get a explosion just like in the first movie. Garraka freezes everyone up and is about to kill Phoebe, but the ghost girl lights up a match, in which Nadeem becomes Raiden all of a sudden and shoots out a fireball. The og team uses the broken containment unit to trap Garraka and seal him away. The theme song plays and there is a crowd just like in GB 1 and 2 chanting Ghostbusters. Peck pulls up, someone in the crowd calls him dikkless and he acknowledges the Ghostbusters as his tribal chiefs.
Post credit scene: The mini puffs steal an 18-wheeler carrying stay puft marshmallows.