Ray Stantz (DAN AYKROYD), Peter Venkman (BILL MURRAY), and Egon Spengler (HAROLD RAMIS), are three scientists deemed a disgrace by the dean of Columbia University and told that their grant has not been renewed and they're no longer welcome. That's most likely because their fascination with paranormal activity (though Venkman seems to be around just to be around) is seen as too weird for any campus to support.
Then, an idea from Venkman: "Professional paranormal investigations and eliminations." Ghostbusters, for short, which starts out in an abandoned firehouse, with no customers, and then Dana Barrett (SIGOURNEY WEAVER) comes to them, wanting to know what paranormal being is living in her fridge, complete with a pointy-headed monster. Word gets around of the Ghostbusters' existence, and the business and publicity flies in fast, even attracting Winston Zeddmore (ERNIE HUDSON), not at all worried about what the business is, as long as it brings in a paycheck.
But the big problem for the Ghostbusters comes when Zuul, the being in Dana's fridge, wants more room than the fridge contains and it turns out that there's yet another being. Then a bothersome bureaucrat from the EPA (WILLIAM ATHERTON) causes even more havoc that brings everything to a head and the Ghostbusters to a challenge literally bigger than anything they could ever have envisioned this line of work bringing forth.