Following the death of her father, neurotic and spoiled heiress Carrigan Crittenden discovers she has only been left Whipstaff Manor, located in
Friendship, Maine, in the will while his vast wealth has gone to several charities. Carrigan and her lawyer and close friend, Dibs, find a map within the will's papers that tell of an alleged treasure hidden inside the manor, only to find that the property is haunted by a friendly ghost named
Casper and his
poltergeist uncles, the
Ghostly Trio. They then unsuccessfully attempt to force the ghosts out by way of
exorcism,
Ghostbusting, and a demolitions team. A lonely Casper watches a news report about paranormal therapist James Harvey and instantly becomes smitten with his teenage daughter, Kat, prompting Casper to inspire Carrigan in summoning James to Whipstaff. Kat dislikes her father's reputation and obsession with contacting the ghost of his late wife, Amelia. The Harveys move into Whipstaff, but Casper's attempt to befriend them fails when they see him. His uncles, the Ghostly Trio, try to torment and scare them away, which eventually fails. The following morning, Casper gains the Harveys' trust when he serves them breakfast, and follows Kat to school, where she becomes popular when her class learns that she is living in Whipstaff, and agrees to host their
Halloween party there. One of her classmates, Amber, plots with her friend, Vic, to humiliate Kat during the party, while James attempts therapy sessions with the Ghostly Trio, who reveal that they know Amelia. In exchange for convincing Carrigan to leave them alone, they promise to get James a meeting with his wife. Kat learns that Casper has no memory of his life, and restores his old playroom in the attic to remind him. Casper recognizes an old wooden
sled his father bought him, and remembers playing outside until he caught a severe cold and died of
pneumonia, becoming a ghost to keep his father company. A newspaper article reveals that Casper's father was declared legally insane after he built a machine, the Lazarus, which he claimed could bring the dead back to life. Casper and Kat venture to the basement and find the Lazarus. Carrigan and Dibs sneak inside and steal the formula that powers the machine, plotting to use it to grant themselves immortality. However, they attempt to kill each other to test the theory and retrieve the treasure that they think is in the basement's locked vault. This culminates in Carrigan attempting to run Dibs over with her
Range Rover, but instead crashing into a cliff-side tree. Upon exiting her car, Carrigan falls to her death and becomes a ghost. James becomes depressed after the trio pulls a prank on him, prompting them to take him out on the town. They plan to kill him and make themselves a quartet, but they have a change of heart after a drunken James confesses how much he cares for them and vows to let them stay in their home. However, James accidentally falls to his death. In the laboratory, Carrigan confronts Casper and Kat, stealing what she believes to be the treasure from the vault and launching Dibs out a window when he tries to double-cross her. Carrigan demands to be brought back to life, but Casper and Kat remind her ghosts are people with unfinished business and now that she has Casper's treasure, her unfinished business is fulfilled, causing her to be ejected into the
afterlife. The treasure is revealed to be Casper's prized baseball, signed by
Duke Snider; the map was part of a game Casper played with his father. James, now a ghost, returns with Casper's uncles. Seeing Kat's despair over her father's death prompts Casper to sacrifice his one chance to return to life, restoring James instead. The Halloween party kicks off upstairs; Amber and Vic's prank is thwarted by the Ghostly Trio, and they flee in terror. In his room, Casper is visited by Amelia, who had become an angel after her death. She rewards Casper for his sacrifice by temporarily turning him into a human boy, allowing him to attend the party and dance with Kat. Amelia meets with James, explaining that the Ghostly Trio kept their promise to get him a meeting with her, and tells him that she was so content with her family while alive that she had no unfinished business and thus moved on to the afterlife instead of becoming a ghost. Amelia departs as the clock chimes ten, promising James that they and Kat will be together again one day. After kissing Kat, Casper transforms back into a ghost, inadvertently scaring the guests away, but Kat is nonetheless impressed with the party. James declares that it is not over yet and the Ghostly Trio play their nephew's theme for them to dance to. ==Cast==