Carrigan Crittenden inherits Whipstaff Manor, an old mansion, from her late father and hears of a hidden treasure in it. She and her assistant Dibbs go into the house but find it haunted. After no luck with various ghost exterminators, Carrigan hires a ghost therapist, Dr. Harvey, who supposedly can help ghosts cross over by resolving unfinished business. Harvey and his daughter Kat move into the mansion and it turns out that Harvey is in this line of work to find the ghost of his recently departed wife.
The ghosts are the gentle Casper and his three naughty uncles, Stretch, Stinky and Fatso. The uncles create mayhem in the house, but grow to like Dr. Harvey while Casper, who's always wanted a friend, hits it off with Kat. Casper as a boy had gotten lost in the woods and froze to death and his father, a great inventor, was working on a machine to bring ghosts back to life called the Lazarus. When Dr. Harvey is taken for a night on the town by Casper's uncles, Carrigan and Dibbs sneak into the house and follow Kat and Casper to the Lazarus and the vault containing the treasure. The advantages of having a ghost who can access the treasure and then come back to life dawn on Carrigan and Dibbs, so they try killing each other, but after one of them becomes a ghost and gets the treasure, a double cross is pulled and neither one gets its so it falls into Kat and Casper's hands.
Meanwhile, Dr. Harvey's night out with the ghosts has him so soused that he falls into an open sewer and kills himself, necessitating the use of the Lazarus on him instead of Casper. In sympathy for their new friend, the uncles call Dr. Harvey's wife from the afterlife and she tells him to get over her death and pay attention to Kat, who is growing up and needs a life-involved parent. She is able to grant Casper's wish to become human again for a little while, so he makes an appearance at her Halloween party, but at the stroke of ten, he reverts back to his old self and literally gets the party off the grounds.