The Ghostbusters' old "pal" and nemesis, Walter J. Peck, ex-EPA hatchet man extraordinaire, is back from the Gozer incident, and he's trying to raise some trouble with the Ghostbusters again. First he tries to get them arrested for illegal trespassing by calling in a phony ghost alert at a military base in Queens, but after that backfires, Peck joins up with BUFO (Bureau of Unidentified Flying Organisms), and takes Slimer with a court order. After some tests, Peck deems Slimer a hazard case and plans to destroy him with a cyclotron, forcing Ghostbusters to save Slimer from Peck.
The Ghostbusters release the Stay Puft Marshmallow Man from the containment unit to shoot a commercial in exchange for getting $50,000 for a children's hospital. However, during the process of letting Stay Puft out, a vicious manta ray-like phantom escapes in order to seek revenge against the Ghostbusters.