The plot centers on Carrigan Crittenden's attempt to retrieve a treasure from a haunted house, Casper's wish to be human again, Kat and Casper's relationship and Dr. James Harvey's search for the ghost of his late wife.
Four direct-to-video sequels to the movie followed. An animated series, in which the ghosts were voiced by the same voice actors as in the movie, aired on American television between 1996 and 1998.
Carrigan Crittenden has inherited an old house named Whipstaff Manor. She and her attorney Paul "Dibbs" Plutzker discover a message written in invisible ink which says that there is a treasure hidden in the house. When they arrive there, they find that the house is haunted by Casper the Friendly Ghost and the unfriendly Ghostly Trio of Stretch. Stinkie and Fatso. Carrigan and Dibbs hire several professionals to drive out the ghosts, including Ray Stantz of the Ghostbusters.
Casper sees a television news report about Dr. James Harvey, a psychologist for the dead who helps ghosts to resolve their unfinished business and move on to the afterlife. He falls in love with the psychologist's daughter Kathleen "Kat" Harvey. He infiltrates Carrigan's hotel room and possesses her television to draw her attention to the news report and indirectly convince her to hire Harvey to investigate the haunting.
It is revealed that Dr. Harvey has been moving around the country searching for the ghost of his late wife Amelia, this creates problem for Kat since she cannot makes friends with kids her age. Once they arrive at the manor, Casper makes an attempt to be friends with Kat. The Ghostly Trio originally try to drive the Harveys out of the house but they grow to like Dr. Harvey and even consider making him a ghost like them. Kat attends the local school. When the other students find out that she lives in the haunted Whipstaff Manor, they decide to hold their upcoming Halloween party there.
Kat finds out that Casper has no memory of his life before he became a ghost. She finds some of his old toys in the attic. When he sees them, his memories come back. He finds his old sled and remembers that he died after playing on his sled all day in the cold. They find a newspaper clipping which says that Casper's father was sent to an insane assylum after trying to build a machine that would bring ghosts back to life.
In an underground laboratory, Kat and Casper find the machine and enough serum to bring one ghost back to life. At the same time, Carrigan and Dibbs arrive, having found out that the treasure is inside a vault in the laboratory. Unable to open the vault door, they decide that one of them should become a ghost to walk through the vault door before using the machine to come back to life. Carrigan and Dibbs can not decide which of the two of them should die and try to kill each other. During their fight, Carrigan accidentally falls to her death, returns as ghost and takes the treasure from the vault. When she asks Dibbs to use the machine to make her human again, he refuses, having grown tired of being bossed around by her. Casper and Kat trick Carrigan into saying that she has no unfinished business, which causes her to disappear into the afterlife. It is revealed that the treasue is an autographed baseball which once belonged to Casper.
The Ghostly Trio and Dr. Harvey return from a drunken night out. Dr. Harvey has died and become a ghost after accidentally falling down a hole in the street. Casper gives up his one chance to become human again and uses the serum and the machine to bring Kat's father back to life. At that point the students from Kat's school arrive for the Halloween party.
Casper is visited by the spirit of Kat's mother Amelia. She rewards Casper's sacrifice by allowing him to become human again until ten o'clock that night. Amelia's spirit also visits her husband and tells him to stop looking for her, she had no unfinished business and has moved on. Casper goes down to the party and dances with Kat. As the clock strikes ten, while he is kissing Kat, he becomes a ghost again. All the guests flee in terror but Kat and her father continue to dance to music played by the Ghostly Trio.
Sequels and spin-offs
The movie Casper revived interest in the character of Casper the Friendly Ghost. As a result, a new animated series, The Spooktacular New Adventures of Casper, was made which originally ran on NBC in the United States between February 1996 and October 1998.
Four direct-to-video movies were made which are usually considered to be sequels to Casper, although they do not refer to the events of the 1995 movie and sometimes contradict it. Like the 1995 movie, the first two of those video sequels, Casper: A Spirited Beginning (1997) and Casper Meets Wendy (1998), combine computer generated animation with live action. Casper Meets Wendy stars a young Hilary Duff as Wendy the Good Little Witch. Casper's Haunted Christmas (2000) and Casper's Scare School (2006) feature computer generated animation only.