The Ghostly Trio (Stretch, Stinkie, and Fatso) have lived in abandon mansion known as Whipstaff Manor for centenaries along with their nephew: Casper McFadden. They spend most of their afterlives haunting the living in Friendship, Maine while also bossing Casper around, and forcing him to do choirs around the house. They happen to be the older brothers of Casper's father J.T. McFadden. It is never clear what has happen to them, and how exactly did they passed away and become spirits.
One day when two intruders: Carrigan Crittenden and Paul "Dibbs" Plutzker, invade in their house in attempt to gain the treasure that was hidden in the mansion, they immediately scared them away due to their scream, when they saw Casper who was trying to greet them. When Carrigan and Dibbs, tried several attempts to get them out of the house such as hiring a priest, a Ghostbuster, and a group of construstion workers to tear down the house, the Trio wound up getting the upper hand, and scared them all away.
When the Trio returned home one night, after a night of haunting, they were being stalled by Casper, in attempt not to scare away their new house guests: paranormal therapist Dr. James Harvey, and his daughter Kathleen "Kat" Harvey, after Casper indirectly, had Carrigan hired Dr. Harvey to examined the house, in order to be friends with Kat. They obviously expect he is up to something, and so they tossed up aside like a rubber band, and preceed to go in. Once they noticed Dr. Harvey, they tried to scare him away which nearly succeed until he pulled out a vaccum and sucked them inside.
The next morning, they intruded on the Harvey's having their breakfast, in which Casper made for them, much to their displeasure to see that their nephew would serve as what Stretch calls "intruders" before his own flesh and blood. After Casper served them, Harvey insist on helping the Trio crossover, but they refuse. They later discovered Harvey's deseased wife Ameila Harvey on a photo and realize that Harvey is looking for her spirit, which the Trio agree to insist on helping only if Harvey gets Carrigan to leave them alone. However, it turned out to be a prank when they had Fatso dress in drags, and give him a big fat kiss on the lips.
The next morning, they noticed that Harvey has given up attempt to pack his things, to leave, much to their dismay as they were beginning to get along, so they decided to take him out to a bar, where he was doing karoke. Believing that Harvey has no future to look forward to they attempt to kill him, and make themselves a quartet, but when a drunken Harvey tells them that they can stay in their homes, and that he loves them, they immediately had a change of heart broke down into tears. However, Harvey accidentally falls to his death in a manhole.
The Trio along with the spirit, of Dr. Harvey returned home much to Kat's shock to see her father is now a ghost with no memory of his daughter, as the Trio were making fun of her. Kat snapped of his amisa when she pull the picky promise they made before they moved.
Casper stepped in, and took Harvey to a machine call the Lazrus that brings ghosts back to life, which Casper was originally going to use on himself, but can't have Kat live her life by losing another parent.
During Kat's Halloween party, the Trio scared away two of Kat's classmates, who were attempt to ruin the party by scaring away their fellow classmates. After Casper unintentionally scared away classmates, when turns back from a human boy to a ghost, after Ameila's spirit granted him a second chance in life up until 10:00, for his selfless actions in bringing her husband back to life, the Trio all dressed like Little Richard and performed the Casper the Friendly Ghost theme as their nephew and the Harveys dance to it.