In Ghostbusters, University parapsychologists Dr. Peter Venkman (Bill Murray), Dr. Raymond Stanz (Dan Aykroyd) and Dr. Egon Spengler (Harold Ramis) lose a research grant when their experiment methodology is proven to be bogus. The team decides to go into business for themselves and open ‘Ghostbusters,’ a ghost removal service. After struggling to get on their feet, they are summoned to investigate the strange happenings in Dana Barrett's (Sigourney Weaver) Central Park West apartment. What they discover is that all Manhattan is being besieged by ghosts and other worldly demons through a portal in her building.
A young boy receives a new "pet" for Christmas called a Mogwai. In order to take care of the furry creature, he must follow three very important rules: Never expose it to bright light (which will kill it), never get it wet (which will make it multiply) and never, ever feed it after midnight.
When the rules are unintentionally broken, it unleashes many of the mischievous monsters across the town.