At New York's Library, a librarian discovers a ghost down in book hold, sometime later, a trio of parapsychologists, Peter Venkman, Raymond Stantz and Egon Spengler, along with Ryan, Winnie the Pooh, Good Fairy, Mewtwo, Ed, and the rest of their friends try to find the only the library ghost, only for it to scare them to retreat. Soon, Peter, Ray and Egon lose their jobs at Columbia University, but with the help of Pooh, his friends and the Ed's, they establish their own paranormal exterminator service, "Ghostbusters." the business, operating out in an abandoned fire station, gets off at a slow start, but just as they run out of money, the Ghostbusters and the heroes are called to Sedgewick Hotel, where there (after a crazy chase, including Peter ending up getting slimed by the pest) they capture their first ghost, soon the Ghostbusters become skyrocketing famous when ghost activity starts rising across New York. The team later hire their new member, Winston Zeddemore, but they are also visited by Walter Peck, who demands to see where they keep the ghosts when captured. The Ghostbusters are hired by Dana Barrett, a woman whose apartment at 55 Central Park West is haunted by an demonic spirit named Zuul, a demigod worshipped in 6000 BC as a servant to Gozer the Gozerian, an Sumerian shap-shifting God. Venkman fall in love with Dana during the search for Zuul in her apartment. As they look into the matter, Dana gets possessed by Zuul, which declares itself as the "Gatekeeper". Louis, Dana's socially-inept neighbor, also discovers and gets possessed by another demon named Vinz Clortho, the "Key Master", who is later taken to the Ghostbusters for checking. The next day things go from bad to worst, Walter Peck returns to shut down the ghost containment grid, doing so causes the grid to explode and the roof of the HQ, ripped open, releasing all the ghosts that were captured to cause chaos across the city, during the havoc the Ghostbusters and the heroes are arrested and Louis/Vinz heads to the apartment of Dana/Zuul.
Later that night, a strange noise is heard across the 100 Acre Wood, when Piglet tries to find what it is, to his surprise (and possibly horror)... Mr. Stay-Puft has returned!!
- Ed, Edd n' Eddy guest star in this movie and somehow they've met Mewtwo before.
- In the bonus ending, Stay Puft is discovered by Piglet. Ryan, Pooh and Tigger hear his roar. But this time, the giant ghost is on Kids World's team. He will make his debut in Kids World's Adventures of Star Wars Episode I: The Phantom Menace.