Egon accidentally gets caught in the Proton Beams along with a ghost. The results are horrifying, causing them to exchange aging processes. Egon begins regressing at an alarming rate and the Ghostbusters must reverse the process before he disappears into nothingness.
Suddenly the fictional spirits of Sherlock Holmes, his associate Dr. Watson and the notorious Moriarty materialize in Manhattan. Moriarty absorbs evil from various modern-day sources, determined to transform into a real, corporal villain.
The Ghostbusters are called to a small New England village to exorcise a recently escaped ancient witch Kestrel. She seeks revenge on various town descendants, including the Spengler family. Undercover in the local all-girls academy, Egon is taken possession of by Kestrel.
Poso, an overstuffed ghost has a goal - to be the ghostfather of all ghosts. He succeeds in kidnapping Janine and Louis and demands a ransom from the Ghostbusters. In order to release the captives, Poso wants the Ghostbusters to surrender their equipment and headquarters.
Peter sells the Ghostbusters' television rights and sets a new live show from Al Capone's tomb. The idea is to fake an appearance of the gangster's ghost at midnight. Sadly, the real ghost of Al Capone appears and pulls them into a spiritual world of gangster ghosts.
Crowley is the chairman of Citizens United Against Halloween. When the Ghostbusters refuse to join forces with him, Crowley by mistake opens the Halloween Door and breaks the underworld's ancient contract of imprisonment.
The Ghostbusters get evicted. While packing up, an accident involving one of the traps sends them to the 1950s, Ray's favorite decade. This creates a dangerous dimensional rift and something big is about to come out. Now what?