Peter Venkman (Bill Murry), Raymond Stantz (Dan Aykroyd), and Egon Spengler (Harold Ramis) are three parapsychologists at Columbia University. Ray and Egon are enthusiastic about ghost hunting, but Peter is a little more skeptical. The three lose their jobs because they fail to provide any real evidence. Peter decides they should go into business as professional paranormal investigators and eliminators. However, their business gets to a slow start as few people take them seriously and they spend all their money developing their equipment.
The first person to go to them is Dana Barrett (Sigourney Weaver). Arriving at her apartment, her eggs begin leaping from their shells and her refrigerator shows a vast space with a creature saying "Zuul". However, Peter is unable to find any readings and unsuccessfully tries to flirt with her. She rejects his advances. After that, the Ghostbusters run out of money, but an up-scale hotel calls them to investigate a disturbance. They successfully (albeit chaotically and destructively) trap their first ghost (Slimer) and business begins booming for them. They hire blue-collar worker Winston Zeddemore (Ernie Hudson) to help with the workload.
Peter gives Dana some info on her case; "Zuul" was an ancient Sumerian demigod who was a minion of "Gozer". He successfully arranges a date. However, just before he arrives, Dana is possessed by Zuul and he rejects her advances. Dana's neighbor, the nerdy Louis Tully (Rick Moranis), becomes possessed by "Vinz Clortho" and he is brought by the police to Egon. The next morning, an overzealous EPA agent named Walter Peck shuts down the Ghostbusters' storage system (believing them to be con artists). This releases hundreds of ghosts who wreck havoc.
The Ghostbusters explain the situation to the Mayor, who ejects Peck from his office. By the time they arrive at Dana's apartment, Dana and Louis have opened the portal for Gozer, who brushes off their proton packs. Calling them to choose a form, Ray chooses the Stay Puft Marshmallow Man. Egon theorizes they can destroy Gozer by sealing the portal; this will require them to "cross the streams" (a very BAD idea). They do so and survive, along with Dana and Louis. They return to the street to a cheering crowd.
Some conservatives believe it has a conservative message: government agencies are bad! Well, the EPA itself is not the problem. It's just one guy who happens to be crazy and think everyone around him is guilty (kind of a conservative thing). In addition, it is the scientists who end up saving the day, a conservative nightmare.