While these events occur the leading lady Dana Barrett (Sigourney Weaver) hires the Ghostbusters to investigate paranormal activity in her Central Park West apartment. With an instant attraction to Dana, Peter decides to head up the investigation. Dana informs Peter that “Zuul” a fictitious demigod is speaking to her from her fridge. After a quick inspection of her apartment without any interaction with Zuul Peter decides to ask her on a date. On the night of their date Peter arrives to find Dana possessed. During the interaction between Dana and Peter, Louis Tully (Rick Moranis) is chased down by a beast and becomes possessed as well. It becomes apparent that the two possessed beings are searching for each other. The police locate Louis and instead of taking him to the police station they drop him at the Ghostbusters Headquarters. Egon quickly realizes that in order to prevent a probable disaster it is necessary to keep the possessed Louis, now calling himself the Keymaster, away from Dana who believes she is the Gatekeeper.

With the rapid success of the Ghostbusters keeping all the ghosts contained becomes a rather large job. The Ghostbusters are soon visited by Walter Peck (William Atherton) who is a agent for the Environmental Protection Agency. Peck comes armed with a court order to shut down the Ghostbuster’s containment unit. With their hands tied the team flees the firehouse and Peck releases a flood of ghosts back into the city. As a result of the outbreak Peck has the Ghostbusters arrested. With out anyone to hold back the Keymaster he is free to roam the streets in search of the Gatekeeper. While the team sits in jail they realize that the building that Dana Battett lives in is designed specifically to summon Gozer, the shape-shifting god of destruction. The mayor summons the Ghostbusters from jail and sets them free to prevent the destruction of New York.

With an escort provided by not only the police but the US army, the team heads to Dana’s building at 55 Central Park West. The Ghostbusters arrive a few minutes too late and both the Keymaster and the Gatekeeper have already met. Both Dana and Louis transform into dog-like beast and open an inter-dimensional gateway allowing Gozer to enter the human world. The Ghostbusters manage to contain Gozer to her portal. Not able to enter the human world Gozer invites the team to choose the face of their demise. Peter orders his team to maintain blank thoughts to prevent Gozer from creating anything. Ray does not follow the order and thinks of the “innocent” Stay Puft Marshmallow Man. An over-sized version of the Marshmallow Man appears and begins to destroy New York. Egon quickly develops a plan to cross their proton beams in order to reverse the particle flow to destroy Gozer’s portal. Just as the Marshmallow Man reaches the top of the building the Ghostbusters complete their plan and cover the area in burnt marshmallow pieces. Both Dana and Louis return to their human form. The Ghostbusters save the city and load up into Ecto-1 and drive away through a cheering crowd followed by Slimmer.